DEVELOPMENT AND EMBR YOLOG Y, 457 



onic period, become widely different and serve for widely 

 different pur^^oses in the adult state. So again it has 

 been shown that generally the embryos of the most 

 distinct species belonging to the same class are closely 

 similar, but become, when fully developed, widely 

 dissimilar. A better proof of this latter fact can not be 

 given than the statement by Von Baer that ** the embryos 

 of mammalia, of birds, lizards and snakes, probably also 

 of chelonia, are in the earliest states exceedingly like one 

 another, both as a whole and in the mode of development 

 of their parts; so much so, in fact, that we can often dis- 

 tinguish the embryos only by their size. In my possession 

 are two little embryos in spirit, whose names I have 

 omitted to attach, ttu] c,i present I am quite unable to say 

 to what class they belong. They may be lizards or small 

 birds, or very young mammalia, so complete is the similar- 

 ity in the mode of f orm.at:on of the head and trunk in these 

 animals. The extremitico, however, are still absent in 

 these embryos. But even if they had existed in the earliest 

 stage of their development we should learn nothing, for 

 the feet of lizards and mammals, the wings and feet of 

 birds, no less than the hands and feet of man, all aiise 

 from the same fundamental form.^^ The larvge of most 

 crustaceans, at corresponding stages of development, 

 closely resemble each other, however different the adults 

 may become; and so it is with very many other animals. 

 A trace of the law of embryonic resemblance occasionally 

 lasts .till a rather late age: thus birds of the same genus, 

 and of allied genera, often resemble each other in their 

 immature plumage; as we see in the spotted feathers in the 

 young of the thrush group. In the cat tribe, most of the 

 species when adult are striped or spotted in lines; and 

 stripes or spots can be plainly distinguished in the whelp 

 of the lion and the puma. We occasionally^ tliough rarel}^ 

 see something of the same kind in plants; thus the first 

 leaves of the ulex or furze, and the first leaves of the 

 phyllodineous acacias, are pinnate or divided like the 

 ordinary leaves of the leguminos£e. 



The points of structure, in which the embryos of widely 

 different animals within the same class resemble each other, 

 often have no direct relation to their conditions of exist- 

 ence. We can not, for instance, suppose that in the eai- 



