DEVELOPMENT OF THE FORMS OF ANIMAL LIFE. 



495 



the blastodermic layers — a process which has 

 been looked upon by all those who have engaged 

 in its study with the greatest interest and ad- 

 miration. And if, by comparing these phenom- 

 ena as observed in individuals belonging to dif- 

 ferent classes 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 

 remarkable resemblance and conformity, we shall 

 be led to conclude that there is a general plan of 

 development proved to extend to the members of 

 considerable groups, and possibly capable of being 

 traced from one group to another. But this is 

 clearly 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 ascertained to belong to 

 them all. The main question, therefore, to be 

 answered is, whether there is or is not a general 

 correspondence between the phenomena of de- 

 velopment and the gradation of type in animal 

 structure upon which anatomists and zoologists 

 are agreed ; and my object will now be to bring 

 rapidly before you one or two of the most marked 

 illustrations of the correspondence, drawn from 

 the early history of development in the higher 

 animals. 



As one of the examples of the earlier phe- 

 nomena of development 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 

 protoveriebrce of embryologists, not corresponding, 

 as might at first be supposed, with the true or 

 actual vertebiae which are formed later, but rep- 

 resenting in an interesting manner transverse 

 vertebral segments of the body, and containing 

 within each the elements of a great part of the 

 structure belonging to the body-wall afterward to 

 be developed, including the true cartilaginous or 

 osseous vertebral arches, and the muscular plates. 



This change, however, belongs to the meso- 

 dermic lamina, and occurs in an elongated thick 

 portion of it, which makes its appearance on each 

 side of the primitive neural canal between the epi- 

 blast and the hypoblast. The transverse cleavage 



is ascertained to commence near what afterward 

 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 for- 

 mation at so early a period of the succession of 

 metameres or series of similar pai'ts, which forms 

 a main characteristic of vertebral organization. 



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 in- 

 quiries. 



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 vertebra. It exists as 

 a primordial structure in the embryo of all ver- 

 tebrates, including man himself, and extending 

 down to the amphioxus, and, according to the 

 remarkable discovery of Kowalevsky in 1866, it 

 is to be found among the invertebrates in the 

 larva of the ascidia. 1 



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 carti- 

 laginous and osseous fishes various gradations 

 of cartilaginous and osseous structures come to 

 surround the notochord and give rise to the 

 simpler forms of vertebral bodies, which undergo 

 more and more distinct development in I he higher 

 vertebrates. In all instances the substance form- 

 ing the vertebral bodies is deposited on the sur- 

 face of or outside the notochord and its sheath, 

 so that this body remains for a time as a vestigial 

 structure within the vertebral bodies of the high- 

 er 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 little short of revolu- 

 tionary in embryological and zoological views, 

 leading as they do to the support of the hypoth- 

 esis that the ascidia is an earlier stage in the 

 phylogenetic history of the mammal and other 

 vertebrates. The analogy between the amphioxus 

 and ascidian larva is certainly most curious and 

 striking as regards the relation of the notochord 

 to other parts, and it is not difficult to conceive 

 such a change in the form and position of the 

 organs in their passage from the embryonic to 

 1 "Memoires de rAcadcmie de St.-Pt-tersbourg-," vol. x. 



