342 CLASSIFICATION OF VERTEBRATES. 



a layer of superficial ectoderm cells, and beneath this the lower 

 layer cells of undifferentiated mesoderm and entoderrn. The 

 blastoderm next exhibits two areas, a translucent central area 

 pellucida and a marginal area opaca. At the edge of the blas- 

 toderm there now appears an elongate depression, the primitive 

 streak (an extremely modified blastopore), the axis of which, 

 corresponding to the future axis of the embryo, lies at right 

 angles to the major axis of the egg. In front of this arise a pair 

 of medullary folds enclosing the medullary groove, the hinder 

 ends of the folds embracing the anterior end of the primitive 

 streak. 



While the embryo is thus being outlined the blastoderm in- 

 creases in size, and soon becomes differentiated into embryonic 

 and extraembryonic portions, the former giving rise to the whole 

 of the embryo, the other to a cellular yolk sac which eventually 

 embraces the whole yolk. At the edge of the embryonic area 

 arises the amniotic fold which closes in over the embryo from 

 all sides, thus enclosing it in a double-walled sac, the inner 

 layer being the amnion, the outer the serosa. While the am- 

 nion is being closed in, the embryo begins to be cut off from 

 the yolk, at last only a narrow yolk stalk connecting the two. 

 The allantois grows out from the alimentary tract behind the 

 yolk stalk. At its base it is small, but it expands distally into 

 a large vesicle. Both yolk sac and allantois have blood-vessels 

 developed in them, and form important organs of nutrition in 

 the broader sense. The blood-vessels of the yolk sac appear 

 first as outgrowths from the omphalomesaraic arteries and veins 

 in the area pellucida ; but they gradually extend over the area 

 opaca, branching and forming a plexus, the function of which is 

 to take up the yolk and carry it into the circulation. The 

 allantoic circulation is respiratory in character. Its vessels are 

 outgrowths from the anterior abdominal vessels of the non- 

 allantoidan vertebrates. The allantois also serves as a reservoir 

 of urinary waste. 



In the general growth of the embryo the most striking 

 feature is the close similarity, until a late stage, with the rep- 

 tiles. The gill slits three or four in number never bear 

 gills, and the appendages in the early stages are distinctly paw- 



