August i6, 1877] 



NATURE 



309 



But whatever may be the first origin of the mesoblast, we 

 know that in the vertebrata this layer, separating from between 

 the other two, and acquiring rapidly by its cell multiplication 

 larger proportions and much greater complexity than belongs to 

 either ectoderm or emlolerm, *pee 'ily undergoes further sub- 

 division and differi-niiati'in in connection with the appearance of 

 the embryonic urgans which arise frooi it, and in this respecr 

 contrasts greatly with the simplicity of structure which remains 

 in the developed parts of the ectodermic and endodermic layers 

 Thus, while the ectoderm supplies the formative materials for the 

 external covering or epidermis, together with the rudiments of 

 the central nervous organs and principal sense-organs, and the 

 endoderm by itself only gives rise to the epithelial lining of the 

 alimentary canal and the cellular part of the glands connected 

 with it, the mesoblast is the source of far more numerous and 

 complex parts, viz., the whole of the true skin or corium, the 

 vertebral column and osseous system, the external voluntary 

 muscles and connective tissue, the muscular walls o( the alimen- 

 tary canal, the heart and blood-vessels, the kidneys, and the 

 reproductive organs thus forming much the greatest bulk of the 

 body in the higher animals.' 



There is, however, a peculiarity in the mode of the earliest 

 development of the mesoblast which is of great importance in 

 connection with the general history of the disposition of paits in 

 the animd body to which I must now refer. This consists in 

 the division of the mesoblast in all but its central part into two 

 laminae, an outer or upper and an inner or lower, and the 

 separation of these by an interval or cavity which corresponds to 

 the space existing between the outer wall of our bodies and the 

 deeper viscera ; and which from the point of view of the 

 vertebrate animals is callel the pleuro-peritoneal cavity, but 

 viewed in the more extended series of animals down to the 

 annuloida, may receive the more general appellation of pleuro- 

 splanchnic or parleto-visceral cavity, or, shortly, the ccelom. 

 Thus, from an early period in the vertebrate embryo, and in a 

 considerable number of the invertebrate, a division of the 

 mesoderm takes place into the somato-pleural or outer lamina, 

 and the splanchno-pleural or inner lumina ; the outer being the 

 seat of formation of the dermal, muscular, and osseous systems 

 — the volunto-motory of Remak ; and the inner of the muscular 

 wall of the alimentary canal, as well as of the contractile sub- 

 stance of the heart and the vascular system generally. 



It is interesting to find that there is a correspondence between 

 the later division of the mesoderm of the higher animals derived 

 from the two primitive blastodermic lamina;, and the original 

 absence of mesodermic structure in the lowest animals, followed 

 by the gradual appearance, first of one layer (the external 

 muscular in the higher ccelenterata), and soon afterwards by the 

 two divisions or laminx with the intermediate ccelom. 



In this account of what may be termed the organised founda- 

 tion of the new being, I have entered into some detail, because 

 I felt that our conception of any relation subsisting between the 

 ontogenetic history of animals and their phylogenetic evolution 

 can only be formed from the careful study of the earliest pheno- 

 mena of embryonic organisation. But, notwithstanding the many 

 difficulties which unquestionably still block the way, I am inclined 

 to think that there is great probability in the view of a common 

 bilaminar origin for the embryo of all animals above the protozoa, 

 and that the vertebrate equally with the invertebrate animals may 

 be shown to possess in the first stages of their blastodermic or 

 embryonic formation the two primitive layers of ectoderm and 

 entoderm. 



To attempt, however, to pursue the history of the develop- 

 ment of animals in detail would be equivalent to inflicting upon 

 you a complete system of human and comparative anatomy. 

 But I cannot leave the subject abruptly without an endeavour 

 to point out in the briefest possible manner the bearing of one 

 or two of the leading facts in embryology upon the general 

 relation of ontogeny and phylogeny. 



We are here brought into the contemplation of those re- 

 markable changes, all capable of being observed and de- 

 monstrated, by which the complex organisation of the body is 



1 If we reserve the words ectoderm and endoderm to designate the two 

 layers of the primary bilaminar blcisloderm, we may apply the terms 

 epihiast and hypoblast to their derivatives after the formation of the 

 mesoderm, and indicate the relations of the whole to the secondary or 

 quadrilaminar blastoderm, by the accompanying Table ; — 



f Ectoderm ... { J^P'^ast ^ 



Primary "-M.^^,!.™ / Soinatopleure ...\ .Secondary 



Mesode 



vEndode 



■" \ Splanchnople 

 ... Hypoblast.... 



Blastoderm. 



gradually built up out of the elementary materials furnished by 

 the blastodermic layers, — a process which has been looked 

 upon by all those who have engaged in its study with the 

 greatest interest and admiration. And if, bv comparing these 

 phenomena as observed in individinN belonging to different 

 cla-ses and orders of animals, it is found not only that they are 

 not different, but on the contrary, that they present features of 

 the most rema'kihle resemblance and conf>rmity, we shall be 

 led to conclude that there is a general plan of development 

 proved to extend to the members of considerable gronps, and 

 possibly capable of being traced from one group to another. 

 But this is cleaily nothing else than another way of stating 

 that there is a similar type of structure pervading the animals 

 of each group, and a probability of a common type being ascer- 

 tained to helong to them all. The main question, therefore, to 

 he ans^vered is whether there is or is not a general correspond- 

 ence between the phenomena of development and the gralation 

 of type in animal structure upon which anatomists and zoologists 

 are agreed ; and my object will now he to hring rapidly before: 

 you one or two of the most marked illustrations of the corre- 

 spondence, drawn from the early history of development in the 

 higher animals. 



As one of the examples of the earlier phenomena of develop- 

 ment I may refer to the change which is perceptible as early as 

 the eighteenth or twentieth hour of incubation in the chick, 

 and which is reproduced in the course of development of every 

 member of the vertebrate sub-kingdom. It consists in the 

 formation of cross clefts on each side of the primitive neural 

 cavity which divide off from each other a number of segments 

 of this wall in the length of the axis of the embryo. At first 

 there are only one or two such clefts ; but they rapidly increase 

 in a backward direction in the body of the embryo, and as 

 development proceeds they extend into the tail itself. These 

 are the frotavertebnc of embryologists, not corresponding, as 

 might at first be supposed, with the true or actual vertebrie 

 which are formed later, but representing in an interesting manner 

 transverse verttbral sfgmmts of the body, and containing within 

 each the elements of a great part of the structure belonging 

 to the body-wall afterwards to be developed, including the true 

 cartilaginous or osseous vertebral arches, and the muscular 

 plates. 



This change, however, belongs to the mesodermic lamina, 

 and occurs in an elongated thick portion of it, which makes its 

 appearance on each side of the primitive neural canal between 

 the epiblast and the hypoblast. The transverse cleavage is 

 ascertained to commence near what afterwards forms the first 

 cervical vertebra, but does not extend into the base of the 

 cranium. And it is most interesting to note in this cleavage the 

 formation at so early a period of the succession of mctaineres 

 or series of similar parts, which forms a main characteristic of 

 vertebral organisation. 



As intimately connected with the formation of the vertebral 

 column, the appearance of the chorda dorsalis, or notochord, 

 presents many points of peculiar interest in embryological 

 inquiries. , j r 



The notochord is a continuous median column or thread of 

 cellular structure, running nearly the whole length of the 

 rudimentary body of the embryo, and lying immediately below 

 the cerebro-spinal canal. It occupies in fact the centre of the 

 future bodies of the vertebrre. It exists as a primordial 

 structure in the embryo of all vertebrates, including man him- 

 self, and extending down to the amphioxus, and, according to 

 the remarkable discovery of Kowalevsky in 1S66, it is to be 

 found among the invertebrates in the larva of the ascidia." 



In amphioxus and the cyclostomatous fishes the notochord, 

 growing with the rest of the body into a highly developed form, 

 acts as a substitute for the pillar of the bodies of the vertebrae 

 no vertebral bodies being developed ; but in cartilaginous and 

 osseous fishes various gradations of cartilaginous and osseous 

 structures come to surround the notochord and give nse to the 

 simpler forms of vertebral bodies, which undergo more anci 

 more distinct development in the higher vertebrates In all 

 instances the substance forming the vertebral bodies is deposited 

 on the surface of or outside the notochord and its sheath, so 

 that this body remains lor a time as a vestigial structure within 

 the vertebral bodies of the higher animals. 



The observations of Kowalevsky with respect to the existence 

 of a notochord in the ascidia, which have been confirmed by 

 Kupfer and others, have produced a change httle short ot 

 I Mem. deVAtad. de St. Petersbmr^.^oX. x. 



