ANA TOM V AND MORPHOLOG Y OF INSECTS. 13 



Yelk Cleavage. — In the animal kingdom generally, the first 

 : change which occurs after impregnation is the division of 

 *the germ, first into two, and afterwards into four, eight, and 

 sixteen, cells. This subdivision continues until a cellular 

 membrane, the blastoderm, is completed. When a food-yelk 

 is present this also undergoes more or less complete segmenta- 

 tion, forming what are known as yelk spherules. 



In insects the segmentation of the food-yelk commences in 

 its interior. This form of segmentation is known as centro- 

 lecithal. It appears to be characteristic of all arthropods. 



The Embryo is developed from the blastoderm, which 

 entirely surrounds the yelk at a very early period. It con- 

 sists at first of a single layer of cells, but afterwards of three 

 distinct layers. The most external of these is the epiblast, 

 the innermost is the hypoblast, and the intermediate layer 

 is the mesoblast. 



The hypoblast encloses a cavity known as the archenteron, 

 and at a later period of development as the mesenteron. 



Recent observations made by Biitschli* and by myself on 

 the fly embryo have shown that the hypoblast and epiblast 

 are at first continuous, and that the archenteron is formed 

 by a dorsal invagination, which opens externally on the dorsal 

 surface of the embryo by a large blastopore (Fig. 2, between 

 dp and h). 



The first traces of an embryo as distinguished from the 

 blastoderm appear as a shallow pit on the ventral surface of 

 the blastoderm, the primitive mouth or stomodaeum, and as a 

 narrow, somewhat opaque, longitudinal streak behind it— the 

 primitive band. 



The primitive band arises from the multiplication of the 

 embryonal cells on the inner surface of the epiblast ; it is 

 distinctly divided into two lateral halves by a median furrow. 

 A second pit soon appears at the posterior end of the primitive 

 band ; it is the proctodseum, and becomes the rectum. 



The stomodaeum becomes deeper as development progresses, 

 and ultimately forms a blind tube, which becomes the fore-gut. 



* Biitschli, ' Gegenbaur. Morph. Jahrbuch,' 1888, p. 170. With figures. 



