DEVELOPMENT OF THE FORMS OF ANIMAL LLFE. 



493 



avoid calling your attention to the extremely in- 

 teresting relation which has been 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 espe- 

 cially by the researches of Haeckel himself, of 

 Kowalevsky, Edward van Beneden, and others, 

 and which has received most efficient support 

 from the investigations and writings of E. Ray 

 Lankester in our own country ; so that now we 

 may indulge the well-grounded expectation that, 

 notwithstanding the many and great difficulties 

 which doubtless still present themselves in recon- 

 ciling 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 for- 

 mation of animals and of man. 1 



In all animals, then, above the protozoa, the 

 ovum presents, in some form or other, the bilami- 

 nar structure of ectoderm and endoderm at a cer- 

 tain stage of its development, this structure re- 

 sulting from a process of segmentation or cell- 

 cleavage ; and there are three principal modes in 

 which the double 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 seg- 

 mentation that the doubling of the layers is pro- 

 duced ; 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 endodermic 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 invertebrates, as also mAmphioxus 

 among the vertebrates, a distinct invagination 

 occurs, while in mammals, as recently shown by 

 Van Beneden's most interesting observations in 

 the rabbit's ovum, and probably also in some in- 

 vertebrates, the cells of the ectoderm gradually 

 spread over those of the endoderm during the 

 progress of segmentation, and thus the endoder- 

 mic comes to be inclosed by the ectodermic layer 

 of cells. 



1 I oupht here to refer to the elaborate memoirs of 

 Prof. Semper on the morphological relations of the verte- 

 brate and invertebrate animals contained in the " Arbeiten 

 aus dem zooIol'. zootom. Institut in 'Wiirzburff,'" 1ST5 and 

 1876, in which the conclusions arrived at do not coincide 

 with the views above stated. 



From the very novel and unexpected observa- 

 tions of Van Beneden, it further appears that 

 from the earliest period in the process of segmen- 

 tation in the mammal's ovum it is possible to 

 perceive a distinction of two kinds of segment- 

 spheres, or cells, 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 up- 

 per of the two first-formed segments, and that the 

 whole of the endodermic cells are the descendants 

 of the lower of the first two segmented cells. 

 This, how r ever, is not an isolated fact belonging on- 

 ly 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 blas- 

 todermic formation. Thus, ectoderm and endo- 

 derm, or the primordial rudiments of the future 

 animal and vegetative systems 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 ve- 

 sicular or holoblastic, through the intermediate 

 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 de- 

 velopmental organization ; but as we rise in the 

 scale of animals we find a new feature appearing 

 in their structure 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 con- 

 stituting the mesoderm, with which, in by far the 

 greater number, is connected the formation of 

 some of the most important bodily structures, 

 such as the osseous, muscular, and vascular sys- 

 tems. 



I will not stop to discuss the very 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 struct- 

 ure, that is, later than the two primary layers of 

 ectoderm and endoderm (corresponding to the 



