70 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



Scolopendra, Polydesmus, Phyllodrumia, and Gryllotalpa (Heymons, Nus- 

 baum, and Fulinski) and Eutermes (Knower). 



Philiptschenko preferred to speak of an upper, or outer (ectoderm), 

 and an inner, or lower, layer, believing that a sharp line cannot be drawn 

 from the further derivatives of the inner layer, i.e., secondary (entoderm 

 and mesoderm) layers. He contended that in Isotoma the unpaired 

 median part of the inner layer is a mixed one, giving rise to both entoderm 

 and mesoderm. 



Thus far the ectoderm is accounted for. The question of the deriva- 

 tion of entoderm and mesoderm now arises. In descriptive embryology 

 of arthropods we are confronted with the difficulty of a terminology in 



Fig. 42. — Germ disk of Peripatopsis. {.an) Anus, {bp) Blastopore, (m) Mouth. {From 

 Snodgrass after Balfour.) 



which one group of writers uses the term "entoderm" for one of the two 

 components of the inner (lower) layer while another group uses it for 

 the yolk cells. Prior to the time of the appearance of a paper by Grassi 

 (1884) nearly all investigators of insect embryology were divided into 

 two groups; either they followed Dohrn (1866) in deriving the mid-gut 

 epithelium from the yolk cells, or they followed Kowalewsky in deriving 

 it from the inner wall (splanchnic layer) of the mesodermic somites. In 

 1884 Grassi demonstrated the bipolar origin of the mid-gut epithelium 

 from the inner (lower) layer in the honeybee. This was followed by a 

 brief paper of Kowalewsky (1886) in which he devised a theory wherein he 

 regarded the insect egg at the time of the formation of the germ layers as 

 comparable to a gastrula so stretched out that the mid-gut epithelial 

 rudiment (mesenteron or entoderm) was pulled into two halves. The 

 stretching of the gastrula resulted in a much elongated blastopore, 

 extending along the whole ventral surface as far as the point at which, 

 later, the proctodaeum develops, and the edges of the furrow forming 



