120 GROWTH 



embryos. Some differences already appear, however, such as a 

 slightly larger nasal region, smaller eye and brain, larger trunk 

 viscera and smaller limbs. 



As a representative of the birds, the chick embryo (Figure 

 65C) likewise shows the same general form and proportions as 

 found in early mammalian embryos. Indeed the head region, 

 including the brain and especially the eyeball, is relatively even 

 larger than in mammals. The enormous head is also character- 

 istic of the reptilian embryo, as seen in the lizard (Figure 65D). 

 This close embryonic resemblance of the birds and reptiles re- 

 calls their phylogenetic relationship, as comparatively recent 

 descendants from a common ancestry. 



Among the amphibia and fishes, the embryonic form departs 

 more widely from the types representing the higher vertebrates. 

 As an example of the amphibia, an embryo mud-puppy (Nec- 

 turus) is shown in Figure 65E. Although the general features 

 are similar to those in the preceding series, the predominance of 

 the head region and neural tube is less marked, and the body 

 flexure is slight. The abdominal region is swollen by the inclu- 

 sion of a large mass of yolk, which is gradually consumed dur- 

 ing later development. Similarly in the embryo of the dogfish 

 (Figure 65F), we find the head enlargement and body flexure 

 less conspicuous than among the higher vertebrates. There is, 

 however, a very distinct ventral flexure in the head region. 



In spite of the obvious differences in details, it is clear that 

 these various classes of vertebrates resemble each other in their 

 early embryonic stages more closely than later as adults. Since 

 all the embryos in Figure 6$ are drawn to approximately the 

 same scale of magnification, it is evident that they are much 

 alike in actual size, although the adults of the corresponding 

 species differ greatly in this respect. From this we may con- 

 clude that in order to attain their adult size either the rate or 

 the duration of later growth must be greater in the larger ani- 

 mals. To attain their adult form, it is likewise apparent that in 

 all cases there must be a shift in the relative intensity of growth 



