3o8 



NA TURE 



[August 1 6, 1877 



With respect to the nature of the blastoderm, the organised 

 celhilar siratum resulting from segmentation, and its relation to 

 the previous condition of the ovum on the one hand, and the 

 future embryo on the other, there is presented to us, by modern 

 research, the interesting view that the blastoderm consists, after 

 completion of tlie segmenting process, of two layers of cells, an 

 outer or upper, usually composed of smaller, clearer, and more 

 compact nucleated cells, named ectoJcnii, or cpiblast, and an 

 inner or lower, consisting of cells which are somewhat larger, 

 more opaque, and granular, but also nucleated, and named 

 • iidcidcrin, or Jiypcblast. 



In the meroblastic ova, such as those of birds, the bilaminar 

 blastoderm is discoid and circumscribed, as it lies on the jolk 

 surface, and only comes to envelope the whole of the food-yolk 

 in the progress of later development ; while in the holoblastic 

 ova, and more especially in mammals, the blastoderm from the 

 first extends over the whole surface of the yolk, and thus forms 

 an entire covering of the yolk known as the " vesicular blasto- 

 derm ; " the space within being occupied by fluid. 



Huxley long ago presented the interesting view that these two 

 layers are essentially the same in their morphological relations 

 and histological structure with the double wall of the body in 

 the simplest forms of animals above the protozoa ; and Haeckel 

 has more recently followed out this view, and supported it by 

 his researches in the Calcareous Sponges, and has founded upon 

 it his well-known Gasl?ira theory. According to this view all 

 animals take their origin from a form of Gastiula. In the simpler 

 tribes, as in the instance of the common fresh-water polype 

 or hydra, they proceed no further than the gastrula stage, 

 unless by mere enlargement and slight differentiation of the two 

 primitive layers of cell, representing the persistent ectoderm and 

 endoderm.i 



If, pursuing this idea, we take a survey of the whole animal 

 kingdom in its long gradation of increasing complexity cf form 

 and structure from the simplest animal up to man himself, we 

 find that all the various modifications of organic structure which 

 present themselves are found, in the history of the individual or 

 ontological development of the different members of the series, 

 to spring originally from two cellular lamina?, ectoderm and 

 endoderm, the component elements of which may again 

 be traced back to the first segment-sphere and primitive 

 protoplasmic elements of the ovum. 



Time does not admit of my conducting you through the chain 

 of obseivation and reasoning by which Haeckel seeks to con- 

 vince us of the universal applicability of his theory, but I cannot 

 avoid calling your attention to the extremely interesting relation 

 which has tieen shown to exist between the primary phases of 

 development of the ovum and the foundation of the blastoderm 

 in very different groups of animals, more especially by the re- 

 searches of Haeckel himself, of Kowalevsky, Edward Van 

 Beneden, and others, and which has received most efficient sup- 

 port from the investigations and writings of E. Ray Lankester 

 in our own country ; so tliat now we may indulge the well- 



The researches now lererred to are those of Auerbach, Butschli, Stras- 

 burger, Hertwig, and Edw. Van Beneden, and the following may be stated 

 as the points in which they mainly agree : — 



The nucleus when about to divide elongates into a spindle-shaped body, 

 becomes irregular and indistinct, acquires a granular disc or zone in the 

 plane of its equator; this divides into two, and each half moves towards the 

 pole of the spindle on its own side, there being radiated lines of protoplasm 

 between the poles and the equatorial disc. 



The disc segments are the new nuclei, and the subsequent division of the 

 cell takes place in the intermediate space. 



Although these observers still differ in opinion upon some of the details 

 of this process, and especially as to the fate of the germinal vesicle, all of 

 them seem to agree that there are two pronuclei or distinct hyaline parts of 

 the yolk protoplasm, a superficial and a deep one, engaged in the formation 

 of the new nucleus, and both Hertwig and Van Beneden are of opinion that 

 the two proceed from different productive elements. 



The radiated structure of the nuclei had been previously recognised by 

 Fol and Flemming, ar.d further observed by Oellacher. 



1. Butschli s researches are published in the Noz: Act. Nat. Cur., 1S73, 

 aud in the Zcitschr. /iir wisscnsch. Zool., vol. xxv. 



2. Auerbach's observations in his Organolo^. Stiidicn, J874. 



3. Strasburger's observations in his memoir " Ueber Zell-biidung und Zell- 

 theilung," Jena, 1875. 



4. Edward Van Beneden's researches, partly in his memoir " On the 

 Composition and Significance of the Egg," &c , presented to the Belgian 

 Academy in 1868, and more particularly in the extremely interesting pre- 

 liminary account of '* Researches on the Development of Mammalia," &c., 

 1875 ; and in a separate paper in the Jourit. of Microscopical Science for 

 April. 1876. 



5. Oscar Hertwig's Memoirs are contained' in the Morplwlog. Jahrbuch, 

 1S75, and his most interesting and novel observations in the same work, 1877. 



' At this place I will only refer to one of the most recent of Haeckel's 

 works, in which the views alluded 10 abnve are fuUv exposed in a series of 

 must interesting memoirs, viz., "Studien zur Gastia:a-'lheoiie,'"Jena, 1877. 



grounded expectation that, notwithstanding the many and great 

 difficulties which doubtless still present themselves in reconciling 

 various forms with the general principle of the theory, we are at 

 least in the track which may lead to a consistent view of the 

 relations subsisting between the ontogenetic, or individual, and 

 the phylogenetic, or race, history of the formation of animals 

 and of man. ' 



In all animals, then, above the protozoa, the ovum presents, 

 in some form or other, the bilaminar structure of ectoderm and 

 endoderm at a certain stage of its development, this structure 

 resulting from a process of segmentation or cell cleavage ; 

 and there are three principal modes in which the doulile 

 condition of the layers is brought about. In one of these 

 it is by inward folding or invagination of a part of the single 

 layer of cells immediately resulting from the process of segmen- 

 tation that the doubling of the layers is produced ; in the 

 second, perhaps resolvable into the first, it may be described 

 rather as a process of inclosure of one set of cells within another ; 

 while in the third the segmented cells arranged as a single layer 

 round a central cavity of the ovum, divide themselves later 

 into two layers. But the distinction of ectodermic and endo- 

 dermic layers of cells is maintained, whether it be primitive 

 and manifested from a very early period, or acquired later by 

 a secondary process of differentiation. Thus, in many inverte- 

 brates, as also in Amfhio.xiis among the vertebrates, a distinct 

 invagination occurs, while in mammals, as recently shown by 

 Van Bcneden's most interesting observations in the rabbit's 

 ovum, and probably also in some invertebrates, the cells of the 

 ectoderm gradually spread over those of the endoderm during 

 the progress of segmentation, and thus the endodermic comes to 

 be inclosed by the eclodermic layer of cells. 



From the very novel and unexpected observations of Van 

 Beneden it further appears that from the earliest period in the 

 process of segmentation in the mammal's ovum it is possible to 

 perceive a distinction of two kinds of segment-spheres, or celln, 

 and that when this process is traced back to its first stage it is 

 found that the whole of the cells belonging to the ectoderm are 

 the progeny of, or result from the division of the upper of the 

 two first formed segments, and that the whole of the endodermic 

 cells are the descendants of the lower of the two first seg- 

 mented cells. This, however, is not an isolated fact belonging 

 only to mammalian development, but one which very nearly 

 repeats a process ascertained to occur in a considerable number 

 of the lower animals, and it seems to promise the means of 

 greatly advancing the comprehension of the whole process of 

 blastodermic formation. Thus, ectoderm and endoderm, or the 

 primordial rudiment? of the future animal and vegetative sys- 

 tems of the embryo, are traced back as distinct from each 

 other to the first stage of segmentation of the germ. 



Accepting these facts as ascertained, they may be regarded as 

 of the deepest significance in the phylogenetic history of animals ; 

 for they appear to open up the prospect of our being able to 

 trace transitions between the earliest embryonic forms occurring 

 in the most different kinds of ova, as between the discoid or 

 meroblastic, and the vesicular or holoblastic, through the inter- 

 mediate series which may be termed amphiblastic ova. 



In the lowest animals, the two layers already mentioned, viz., 

 ectoderm and endoderm, are the only ones known to constitute 

 the basis of developmental organisation ; but as we rise in the 

 scale of animals we find a new feature appearing in their struc- 

 ture which is repeated also in the history of the formation of 

 the blastoderm in the higher animals up to man. This consists 

 in the formation of an intermediate layer or layers constituting 

 the mcsoiierm, with which, in by far the greater number, is con- 

 nected the formation of some of the most important bodily 

 structures, such as the osseous, muscular, and vascular systems. 



I will not stop to discuss the veiy difficult question of the first 

 origin of the mesoderm, upon which embryologists are not yet 

 entirely agreed, but will only remark that a view originally taken 

 of this subject by the acute von Baer appears more and more to 

 gain ground ; and it is this — that the mesoderm, arising as a 

 secondary structure, that is, later than the two primary layers of 

 ectoderm and endoderm (corresponding to the serous and mucous 

 layers of Pander), is probably connected with or derived from 

 both of these primitive layer.s, a view which it will afterwards 

 appear is equally important ontogenetically and phylogenetically. 



^ I ought here to refer to the elaborate memoirs of Prof Semper on the 

 morphological relations of the vertebrate and invertebrate animals contained 

 in the "Arbeiten aus dem zoolog. zootom. Institut in Wiirzburg,'* T875 and 

 1876, in w^ich the conclusions arrived at do not coincide with the views 



