138 WILLIAM BATESOJJ. 



The segmentation cavity (s. c.) is a large empty space; and 

 neither at this^ nor at any subsequent period of development 

 do any mesenchyme cells appear in it, the mesoblast arising 

 entirely from the walls of the archenteron, as will be after- 

 wards described. 



By a progressive diiferentiation the plano-convex form shown 

 in fig. 19 is reached. At this period the disparity in size 

 between the cells of the two regions is still more marked, and 

 no part of the wall is composed of indifferent cells. 



Figs. 20 and 21 represent the appearances seen in transverse 

 sections of rather older Gastrula^. No histological change of 

 importance has taken place. In fig. 22 the fusion of the 

 layers which accompanies the closing of the blastopore is 

 shown. This region of fusion is seen in a series of sections to 

 be somewhat longer in one direction than in the other, con- 

 formably with tbe external characters of Stage A. 



In Stage B the blastoporic fusion still persists, but the 

 hypoblast is nearly separated from the epiblast, only remaining 

 connected with it for a very small area, "which marks the point 

 at which the blastopore finally closed. When Stage E is 

 reached this area is still further reduced, but can be traced 

 until Stage D. 



In a section through the anterior end of this stage, the 

 epiblast is seen to be composed of large cuneiform cells con- 

 taining granules, whose outer ends are covered with very 

 minute cilia. Between their internal ends spaces are shown, 

 which are no doubt due to contraction of the protoplasm 

 caused by reagents ; similar spaces are also observable between 

 the cells of the hypoblast. At the front end of this larva 

 these latter take a more amoeboid character than in the middle 

 and posterior regions (fig. 25, H'). 



The epiblast is closely applied to the hypoblast in the 

 anterior half of the larva, causing the segmentation cavity to 

 be obliterated. The hypoblast of the middle region is seen to 

 be more columnar in character than that of the anterior 

 region, while the other appearances are the same (figs. 23 

 and 25). 



