250 



INVEETEBEATA 



CHAP. 



in reality only a secondary modification of the ordinary columnar 

 ectoderm. 



The amniotic fold is not a simple indentation at right angles 

 to the germinal area; on the contrary, it has a peculiar trilobed 

 growth, and the indentation is prolonged into a median and two 

 lateral pockets. This, however, is a feature peculiar to Donacia, and 

 this trilobed appearance disappears as the posterior amniotic fold 

 advances forward over the germinal area. 



Whilst this is going on, the median field of this area becomes 

 markedly depressed beneath the surface, so as to form an elongated 



gastral groove (gast.gr, Fig. 

 198). The floor of this 

 groove is composed of cells 

 in a condition of active 

 proliferation, and it forms a 

 wedge-shaped mass in the 

 hinder end of the groove ; 

 in the middle the groove is 

 deepest and its cavity largest, 

 whilst in front it is very 

 shallow. The two sides of 

 the groove meet, and it 

 becomes thus completely 

 closed off from the exterior. 

 Throughout most of its 

 extent this overgrowth takes 



am * place in such a way that 



the cavity of the groove is 



FIG. 198. Two transverse sections through the quite obliterated, but in the 



* sipes hinder region the gastral 

 groove forms a hollow tube, 

 which can be seen for some 



after it is closed. (After Hirschler.) 



A, section through anterior region where the amniotic 

 folds have not yet met one another. B, section through 



posterior region where the amniotic fold covers the blasto- time lying Under the 6CtO- 

 derm. Letters as in previous figures. In A the gastral ^p-pj-p ' 

 groove is a solid cord of cells, in B it is a tube. 



At the same time also 



an anterior amniotic fold is formed, and grows backwards to meet 

 the advancing posterior amniotic fold. The two meet and fuse, the 

 inner or amniotic limb of one becoming continuous with the amniotic 

 limb of the other, and the outer limb or serosa of the one joining 

 the outer limb or serosa of the other. 



Then the embryonic area begins to show the first signs of 

 segmentation. The front end has become bilobed, and these two 

 lobes correspond to the cephalic lobes of the crayfish. The transverse 

 lines which indicate the division of one segment from the next, do 

 not appear in regular order from before backward, but a few appear 

 before the others and then the rest are, so to speak, intercalated 

 between these. Hirschler attaches great importance to this pheno- 

 menon, and speaks of the embryo becoming at first divided into 



