DEVELOPMENT. 



183 



Since the yolk of the newty-laid egg of the Cockroach is of 

 a consistence extremely unfavourable to hardening and micro- 

 scopic investigation, I have not been able to obtain transverse 

 sections of the germinal vesicle, nor to study the mode of its 

 division (segmentation). If, however, we may judge from 

 what other observers have found in the eggs of Insects more 

 suitable for investigation than those of the Cockroach, we shall 

 be led to conclude that a germinal vesicle, with a germinal spot 

 surrounded by a thin layer of protoplasm, lies within the nutri- 

 tive yolk of the Cockroach egg. From this protoplasm all the 

 cells of the embryo are derived. 



The germinal vesicle, together with the surrounding proto- 

 plasm, undergoes a process of division or segmentation. Some 

 of the cells thus formed travel towards the surface of the egg 

 to form a thin laver of flattened cells investing the volk, the 



*/ o %j 



so-called blastoderm, while others remain scattered through the 

 yolk, and constitute the yolk-cells (fig. 107). 



On the future ventral side of the embryo (and therefore on 

 the concave surface of the egg) the cells of the blastoderm 

 become columnar, and here is formed the so-called ventral 

 plate, the first indication of the embryo. This is a long narrow 

 flattened structure (fig. 104). It is wider in front where the head 



LIBRARY ~ 



I ~*> 



Fig. 104. Ventral Plate of Blatta germanica, with developing appendages, seen 



from below, x 20. 



segment is situated ; further back it becomes divided by manv 



*- / 



transverse lines into the primitive segments. The total number 

 of segments in the ventral plate of Insects is usually seventeen.* 



* Balfour, Embryology, Vol. I., p. 337. 



