5G THE FIRST DAY. [CHAP. 



The vertebral plate, while still continuous with the lateral plate (the dis- 

 tinction between the two being indicated solely by the cleavage of the latter), 

 consists of several layers of cells ; but of these only the uppermost layer, that 

 immediately under the epiblast, appears to be cmitinued into tlie somatopleure ; 

 the whole of the remainder, including those cells which will eventually form the 

 so-called nucleus of the protovertebrte, seem to pass directly into the splanchuo- 

 pleure. 



All these changes except the formation of the pleuro-peri- 

 toneal cavity can be seen in surface views of fresh trans- 

 parent specimens, but their nature is best shewn in sections. 



12. Since the commencement of incubation the area 

 opaca has been spreading outwaixls over the surface of the 

 yolk, and by the end of the first day has reached about the 

 diameter of a sixjoence. It appears more or less mottled 

 over the greater part of its extent, but this is more particu- 

 larly the case with the portion lying next to the pellucid 

 area ; so much so, that around the pellucid area an inner ring 

 of the opaque area may be distinguished from the rest by 

 the difference of its aspect. 



At about the 20th — 24th hours an increasing number of 

 formative cells make their way from the segmentation-cavity 

 to the edge of the area opaca, and there, immediately under- 

 neath the epiblast, quickly become converted into a rather 

 thick and somewhat irregular network of mesoblast cells. The 

 mottled appearance of the inner ring spoken of above is due 

 to changes taking place in this mass of mesoblast, changes 

 which eventually result in the formation of what is called the 

 vascular area, the outer border of which marks the extreme 

 limit to which the mesoblast extends. 



During the whole of this period the medullary groove has 

 been growing rapidly backwards, so that the primitive groove 

 appears to be pushed further and further back, and at the 

 same time becomes smaller and less conspicuous. The 

 amniotic fold is at the end of the first day very noticeable. 



13. The changes then which occur during the first day 

 may thus be briefly summarized : 



(1) The hypoblast and mesoblast are formed from the 

 segmentation-spheres, so that by the 6th to the 8th hour the 

 three layers of the germ — the epiblast, the mesoblast, and the 

 hypoblast — are definitely established. 



(2) The primitive streak is formed by a thickening of 

 the mesoblast. 



