vii THE EXTERNAL FEATURES OF THE BODY 439 



yolk upon the growth in length of the ventral side of the body is 

 much more marked than in other Vertebrates. The result is a 

 strong ventral curvature of the body. The ventral flexure of the 

 head in the mid-brain region already seen in the Elasmobranchs 

 and G-ymhophiona is here still more marked, but in addition the 

 whole body is strongly curved ventrally to such an extent as to form 

 more than one complete turn of a spiral. This curvature is in its 

 incipient stages of great morphological interest as providing a possible 

 explanation of the downward indentation of the blastoderm by the 

 head and tail regions, and their consequent ensheathment in blasto- 

 dermal pockets, which led eventually to the evolution of the amnion. 



The formation of the amnion and the separation of true amnion 

 from false amnion involve as will be gathered from Chapter VIII. 

 a solution of continuity of the somatopleure or original body-wall 

 and probably this has initiated what is perhaps the most striking 

 feature of amniote development — the loss, at the time of hatching, 

 of a relatively large proportion of somatopleure together with the 

 allantois. 



As a matter of minor detail it should be mentioned that in the 

 case of Birds as compared with the lower Amniota the embryo is dis- 

 tinguished during a long period by the relatively enormous size of the 

 head. It seems reasonable to regard this as in the main an anticipa- 

 tion of the great development of the eyes and optic lobes in the adult. 



In the foregoing short sketch the author has confined himself to 

 the main branches of the Vertebrate stem. He has omitted all 

 reference to three different types near the base of that stem, 

 namely Amphioxus, the Lamprey, and the Myxinoid, which are 

 of great interest in themselves but which are of less importance^ 

 for enforcing general principles of Vertebrate development. Of the 

 three types the first will be found fully described by MacBride in 

 Vol. I. and it will easily be seen how in the general form of body, as 

 in various other characteristics, the young Amphioxus has diverged 

 widely from the more typical Vertebrates. The Myxinoids, so far as 

 they are known from Bashford Dean's researches on Bdellostoma 

 (1899), appear also to be highly specialized. The Lampreys on the 

 other hand have diverged to a much less extent from the normal. 

 The most striking features during the early development of the body 

 are (1) that the head-region, as was the case in Ceratodus and Hypo- 

 geophis and as is the case also with Bdellostoma, shows a marked 

 activity in its growth in length, the mass of yolk persisting longest 

 posteriorly and (2) that the outgrowth of the tail is delayed until a 

 comparatively late period. A marked negative feature in all three 

 of the types mentioned is the absence of all trace of paired limbs. 



(2) The Median Fins. — The primitive Vertebrate, with its 

 segmented musculature arranged along the two sides of the body 

 and its skeletal axis and central nervous system lying at the mesial 

 plane, is clearly a creature constructed for swimming by lateral 



