186 



THE COCKROACH : 



As to the origin of the mesoblast most observers have found* 



O 



that a long groove (the germinal groove] appears in the middle 

 line of the ventral plate (fig. 108), which bulges into the 



Fig. 108. Diagram to illustrate the formation of the Germinal Layers. 



E, epiblast ; M, mesoblast. 



yolk, gradually detaches itself from the epiblast, and completes 

 itself into a tube. The lumen of this tube soon becomes filled 

 with cells, and the solid cellular mass thus formed divides into 

 two longitudinal tracts, which lie right and left of the middle 

 line of the ventral plate beneath the epiblast, and are known as 

 the mesoblastic bands. In the Cockroach I was able to satisfy 

 myself that in this Insect also, the mesoblast, in all probability, 

 arises by the formation and closure of a similar groove of the 

 epiblast. M (fig. 108) represents the stage in which the lumen 

 of the groove has disappeared, and the mesoblast forms a solid 

 cellular mass. 



The origin of the hypoblast in Insects has not as yet been 

 clearly determined. Two quite different views on this subject 

 have found support. Some observers (Bobretsky, Graber, and 

 others) maintain that the hypoblast originates in the } 7 olk-cells, 

 which form a superficial layer investing the rest of the yolk. 

 Others (especially Kowalewskyf ) believe that the process is 

 altogether different. According to the latest observations of 

 the eminent embryologist just named, upon the development of 

 the MuscidcB, the germinal groove gives rise, not only to the 



* Kowalewsky in Hydrophilus, Graber in Musca and Lina, Patten in Phryganidce, 

 myself in Meloe, &c. 



t Biolog. Centrablatt. Bd. VI., No. 2 (1886). 



