34 READINGS IN BIOLOGICAL SCIENCE 



in their subsequent course, they diverge more and more widely from one 

 another. And it is a general law, that, the more closely any animals re- 

 semble one another in adult structure, the longer and the more intimately 

 do their embryos resemble one another; so that, for example, the embryos 

 of a Snake and of a Lizard remain like one another longer than do those 

 of a Snake and of a Bird; and the embryo of a Dog and of a Cat remain 

 like one another for a far longer period than do those of a Dog and a 

 Bird; or of a Dog and an Opossum; or even than those of a Dog and a 

 Monkey. 



Thus the study of development affords a clear test of closeness of struc- 

 tural affinity, and one turns with impatience to inquire what results are 

 yielded by the study of the development of Man. Is he something apart? 

 Does he originate in a totally different way from Dog, Bird, Frog, and 

 Fish, this justifying those who assert him to have no place in nature and 

 no real affinity with the lower world of animal life? Or does he originate 

 in a similar germ, pass through the same slow and gradually progressive 

 modifications, — depend on the same contrivances for protection and nutri- 

 tion, and finally enter the world by the help of the same mechanism? The 

 reply is not doubtful for a moment, and has not been doubtful any time 

 these thirty years. Without question, the mode of origin and the early 

 stages of the development of man are identical with those of the animals 

 immediately below him in the scale: — without a doubt, in these respects, 

 he is far nearer the Apes, than the Apes are to the Dog. 



The Human ovum is about ^25 of an inch in diameter, and might be 

 described in the same terms as that of the Dog. It leaves the organ in which 

 it is formed in a similar fashion and enters the organic chamber prepared 

 for its reception in the same way, the conditions of its development being 

 in all respects the same. It has not yet been possible (and only by some 

 rare chance can it ever be possible) to study the human ovum in so early 

 a developmental stage as that of yelk division, but there is every reason 

 to conclude that the changes it undergoes are identical with those ex- 

 hibited by the ova of other vertebrated animals; for the formative ma- 

 terials of which the rudimentary human body is composed, in the earliest 

 conditions in which it has been observed, are the same as those of other 

 animals. 



Indeed, it is very long before the body of the young human being can 

 be readily discriminated from that of the young puppy; but, at a tolerably 

 early period, the two become distinguishable by the different form of their 

 adjuncts, the yelk-sac and the allantois. The former, in the Dog, becomes 

 long and spindle-shaped, while in Man it remains spherical: the latter, in 

 the Dog, attains an extremely large size, and the vascular processes M^hich 

 are developed from it and eventually give rise to the formation of the 

 placenta (taking root, as it were, in the parental organism, so as to draw 

 nourishment therefrom, as the root of a tree extracts it from the soil) 



