GASTRULATION, FORMATION OF GERM LAYERS 



69 



the edges of the lateral plates separate from the middle plate and then 

 grow together beneath it, the middle plate being thus cut off from the 

 lateral plates to form, at first, a single-layered inner germ layer. In still 

 other forms, as among Orthoptera and some Lepidoptera, the cells along 

 the middle part of the germ band, without forming a distinct middle 

 plate and either with or without the formation of a gastral furrow, 

 multiply by horizontal (tangential) division and give off cells from their 

 inner ends that constitute the inner layer (Fig. 4 IF). The method of 

 inner layer formation is probably of no great phylogenetic significance. 



D E F 



Fig. 41. — Types of gastrulation. {am) Amnion. (ec<) Ectoderm, (il) Inner layer. Qp) 

 Lateral plate, (mp) Middle plate, (ser) Serosa. 



since different types may be found in different sections of the germ band 

 of a single individual, as in the Siphonoptera (Kessel, 1939) and other 

 forms. 



Among some of the more primitive arthropods gastrulation is not 

 accompanied by the formation of a furrow. The furrow in insects appears 

 to be merely an ontogenetic adaptation which may be present or absent 

 in closely related forms. In Isotoma, according to Philiptschenko (1912), 

 gastrulation consists of multipolar migration under the entire blastoderm 

 with the invagination and other gastrulation processes derived from it. 

 In agreement with Metschnikoff (1866) he believes this to be the more 

 primitive, the formation of a furrow being derived from the primitive 

 form. Examples of multipolar gastrulation are found in Collembola, 



