412 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



tion. Later they closely approach each other and are completely 

 separated from the ectoderm. On the eleventh day a second molt occurs. 

 Within the yolk, which still is present in great quantity in the body 

 cavity, there are present a number of cells remaining over after the forma- 

 tion of the mesodermic median layer and the mid-gut epithelium. These 

 cells eventually give rise to the circulatory system and to the muscles 

 of the segments. The Malpighian tubules grow out of the proctodaeum 

 on the twelfth day. Later on in the day the animal hatches with only 

 the rudiments of its appendages present. 



siom 



Fig. 369. — Julus terrestris. Longitudinal section of 11-day-old embryo. (6r) Brain. 

 (ent) Entoderm, {ggl) Ganglion, (mes) Mesoderm, (n) Nerve, {stom) Stomodaeum. 

 {From Heathcote.) 



Polydesmus ahchasius Attems 



The little oval egg of this species, which is scarcely 0.3 mm. long, was 

 collected in the Black Sea region of the Caucasus. The egg undergoes 

 total, equal cleavage (Fig. 370), as is the case with the species of the 

 genera Julus (Schizophyllum) , Strongyloma, and Polydesmus (Polyxenus) 

 observed by Metschnikoff (1874). In the earher stages of segmentation, 

 as described by Lignau (1911), some cleavage cells return into the yolk, 

 so that when there are about 50 peripheral blastomeres there are from 

 4 to 7 in the yolk. Hereupon, by reason of the migration of the nucleated 

 protoplasmic masses which are forced to the periphery, the segmented 

 structure of the egg is no longer evident. Here by further division they 



