10 INTRODUCTORY I 



small yolk-granules disposed in about four layers in the roof 

 of the segmentation cavity, the vegetative hemisphere of large 

 cells with large and abundant yolk-granules in the floor of the 

 segmentation cavity. The distinction between the two regions 

 is not however abrupt, but gradual: about the equator are 

 cells of intermediate size and structure. The superficial animal 

 cells receive the pigment. The segmentation cavity lies in the 

 animal hemisphere. Apart from the formation of this cavity 

 there has been no differentiation during cleavage : the unlike 

 material of the ovum has merely been cut up into unlike 

 pieces, the characters of which in each region depend directly 

 on the structure of that region in the unsegmented egg. But 

 after cleavage comes differentiation, and the first step in this 

 is the formation and closure of the blastopore, during which the 

 material for the germinal layers is laid down, and a new cavity, 

 the archenteron, developed (Fig. 4). The blastopore is formed 

 and closed bilaterally, since the dorsal lip appears first (on the 

 side of the grey crescent, just below the equator), the right 

 and left lateral lips next, the ventral lip last, and since the 

 overgrowth of the fold of pigmented cells is most rapid at the 

 dorsal, least rapid at the ventral, and at an intermediate rate 

 at the lateral lips in between. The archenteron, the cavity 

 developed between the blastoporic fold and the yolk which it 

 covers up, is therefore also most extensive below the dorsal 

 lip, that is, anteriorly. As the yolk-cells are pushed into the 

 segmentation cavity the latter becomes obliterated, while the 

 archenteron is enlarged. From its thin roof are formed 

 the notochord, the dorsal mesoderm, and the roof of the gut ; 

 from its thick floor (the yolk-cells), the ventral mesoderm and 

 the floor of the gut. 



The blastopore, which has meanwhile been reduced to a 

 small circle, now closes by approximation of its lateral lips, 

 the yolk-plug being withdrawn. The nervous system arises 

 in the form of the pear-shaped medullary plate of thickened 

 ectoderm upon the dorsal side. The medullary folds rise up 

 along the edges of this plate, bounding the medullary groove, 

 which by the coalescence of the folds becomes converted into 

 the closed canal of the central nervous system. Gill-plates 



