Ventral Plate 



i6i 



the ' superficial cleavage ' supposed to be characteristic 

 of Arthropoda generally. The superficial cleavage of the 

 insect-egg is however, as Carriere (1897) remarks, only 

 apparent. The cells appear in the interior of the egg, 

 and merely become superficial by migration. 



Some of them, whose function is probably digestive, 

 can be seen at a much later time in stained sections as 

 small, branching, nucleated, protoplasmic masses scattered 

 through the yolk. About twenty 

 hours after egg-laying the sexual 

 germs re-enter the egg (fig. 120), 

 apparently forcing a passage in 

 mass through the hinder end of 

 the blastoderm. 



The cells of the blastoderm now Vcntrai 



. - plate. 



divide rapidly along what will 



afterwards be the ventral surface 

 of the embryo. There is thus 

 formed a thickening, the ventral 

 plate or germ-hand [Keimstrelf), 

 which runs nearly round the egg- 

 lengthwise ; it becomes unusually 

 solid in the region of the future 

 tail (fig. 122). As the ventral plate 



Fig. 120. — Longitiidinal sec- 



gemis^ tS'T^w^^ncinded! thickcns, the cclls ou the dorsal 



and the blastoderm is closed. 

 2/, yolk. 5?, blastoderm, sg, 

 sexual germs. 



fold. 



surface thin out. 



A longitudinal ventral infold- ventrai 

 ing next appears, which deepens rapidly, and the 

 median blastodermic cells are thereby pushed a little 

 way into the yolk. The cavity of the fold is obliter- 

 ated very early, and the infolded cells are cut off from 

 the surface by the reunion beneath them of the outer 

 layer. 



The ventral groove is generally taken to be the cavity of 

 a shallow and elongate gastraea ; this identification only 



M 



