ANIMAL BIOLOGY. [Part I. 



that was, so to speak, begun in the frog. In our illustrative 

 case, we saw the hypoblast produced entirely by invagination. 

 In the frog we saw it partly produced by invagination, partly 

 by differentiation of cells which lie beneath the epiblast. Here 

 we find the hypoblast entirely formed by such differentiation, 

 apparently without any invagination. 



At first the blastoderm covers only a small area. But it 

 gradually spreads outwards in all directions, creeping over the 

 yolk beneath the vitelline membrane, until at last (and this does 

 not take place until comparatively late in development) it 

 envelopes the whole mass of yolk. 1 This is similar to what we 

 saw in the frog ; but it is carried further. The investment of 

 the yolk is complete. There is no circular patch of yolk cells 

 left uncovered to mark the position of the blastopore. Thus the 

 blastoderm creeps over the whole surface. But the embryo is 

 entirely formed in the small area pellucida. 



At the hinder end of this area a linear opacity makes its 

 appearance. This is known as the primitive streak. After a 

 while it becomes grooved (Fig. 35, iii., pr. g.). It is produced 

 by a thickening of the blastoderm and the formation of rounded 

 cells which grow downwards, and soon spread outwards in wing- 

 like masses at the sides of the primitive groove. This is shown 

 diagrammatically in Fig. 35, iv. These lateral masses are hence- 

 forth to be regarded as mesoblast. The process by which they 

 are formed is not dissimilar to that by which the mesoblast is 

 formed in the frog at the inturned lip. There we saw a hypo- 

 blast and a mesoblast layer thus formed. Here we find that 

 only the mesoblast is thus produced. This takes place in the 

 hinder third of the area pellucida. 



In the anterior two-thirds the mesoblast is formed in a 

 different way. Here (as seen in Fig. 35, iii. and v.) the 

 medullary groove is beginning to appear. The hypoblast here 

 breaks up into two layers a flattened lower layer, which retains 

 the character and name of hypoblast, and a more indefinite 



1 The hypoblast and mesoblast are said to be here formed mainly through 

 the instrumentality of the nucleated protoplasm of the germinal walls at the 

 edge of the area opaca. 



