412 AN INTRODUCTION TO ZOOLOGY 



composed of the retained trophoblastic ectoderm plus the entoderm, 

 a circular area is formed of embryonal ecto and entoderm. In other 

 words, then, in this form the embryonal ectoderm becomes exposed 

 on the surface of the blast ocyst. Embryos of this type also develop 

 an amnion, but later and by means of the overgrowth of folds much 

 in the same way as in the chick, as we shall see below. 



The embryonal shield now grows and becomes first oval and then 

 pear-shaped. While it is still oval a primitive streak and primitive 

 knot make their appearance almost simultaneously as an opaque 

 line and spot occupying the posterior two-thirds of the shield. A 

 primitive groove also appears, and about the same time a less obvious 

 dark line running forwards from the knot indicates the presence of 

 a notochordal process. All these structures recall the similar ones 

 in the chick, which they resemble in origin and structure. The 

 primitive streak is the result of the active proliferation of the ecto- 

 dermal cells along a median line, and the cells budded off from it 

 become arranged to form a layer between ecto- and entoderm. As 

 in the chick, then, the mesoderm is ectodermal in origin. The primi- 

 tive knot in the mammal is slightly different from that in the chick. 

 In the first place the entoderm is not firmly united to it on the under 

 side, and, secondly, it may have a perforation which leads to a small 

 cavity in the notochordal process known as the notochordal canal. 

 This structure, while not appearing in the chick, is characteristic of 

 certain Reptiles. In the rabbit, however, this canal is only repre- 

 sented by a groove on the lower surface of the shield. 



It is not necessary to go into the details of succeeding stages of 

 development, since they resemble those of the chick fairly closely. 

 A medullary plate is formed whose edges bend up as medullary folds 

 and finally fuse in the middle line to form a neural tube whose 

 primary brain vesicles are already indicated before the union of the 

 folds. The mesoderm sheet spreads out and soon passes out into 

 the extra-embryonal regions. Paraxial bands develop in it, and 

 segment into typical mesoblastic somites. The coelom appears as 

 a split separating the mesoderm into somatic and splanchnic layers, 

 and is divisible into a myoccel and splanchnoccel. 



With this we may conclude the account of the early develop- 

 ment of the fertilised ovum in four distinct types, Amphioxus, 

 Rana, Callus and a Mammal, Lepus. In spite of the great differences 

 due to the particular conditions under which each individually 

 develops, there is a still greater fundamental similarity which is all 

 the more striking since it is exhibited under such diverse conditions. 



