68 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



Philiptschenko (1912) ventured the opinion that had writers in the 

 past adopted the plan of recognizing only the two primary germ layers 

 (ectoderm and inner, or lower, layer) and then beyond this point treating 

 only of the primitive anlagen of the inner layer, much confusion in descrip- 

 tion would have been avoided. He thus would go back to the old concept 

 of Von Baer of two primary layers which are further divided into funda- 

 mental (or primitive) organs. By this he does not mean to imply that the 

 germ-layer theory is superfluous but, on the contrary, considers it as one 

 of the most useful of all embryological generalizations so far as the 

 homologies of the ectoderm and the inner (lower) layer are concerned. 

 As for the mesoderm and the so-called "secondary entoderm," the two 

 derivatives of the inner layer, these, as at present interpreted, he main- 

 tains are not homologous throughout the animal kingdom. Meisen- 

 heimer (1917) has made a somewhat similar observation. 



Some writers on insect embryology have objected to the use of the 

 term "gastrula" in cases where the invaginated cells are said not to give 

 rise to the entoderm. To this Patten (1884) replied: 



I cannot see that it would influence the use of the term in this case, for it 

 seems to me that an inherited tendency to produce an invagination, the sole 

 object of which is to cause a differentiation into germ layers, however successful 

 or unsuccessful the attempt may be should receive the name that was originally 

 applied to the more simple condition, always holding in mind, however, that it is 

 a condition modified by various agents, the nature of which may or may not be 

 known. 



In the following discussion the terms " gastrulation " and ''gastrula" 

 will be used in the customary manner even in cases where authors have 

 denied that the mid-gut epithelium is an entoderm derivative. 



Among the pterygotes, with some exceptions, gastrulation, as usually 

 interpreted by embryologists, takes place soon after the germ band is 

 formed. The middle part of the band sinks inward and may become 

 separated from the part on either side by a slight ridge. There is thus 

 formed a middle plate and the lateral plates (Fig. 41^,mp.Zp). The lat- 

 eral plates, except for the amniotic (lateral) edges, will form the ectoderm; 

 the middle plate, the strictly internal organs of the future insect. In the 

 more generalized case, as has already been described on page 13, the mid- 

 dle plate continues to sink inward, forming a deep median furrow which 

 is then converted into a sunken tube by the approach and union of the 

 lateral plates beneath it (Fig. 415). The tube now flattens out into a 

 continuous two-layered sheet of cells forming the so-called "inner," 

 or "lower," germ layer (Fig. 41C,tZ). This type of inner layer formation 

 is common, among flies and beetles (Fig. 329^). In other cases, as among 

 certain Lepidoptera and Hymenoptera (Figs. 4:1D,E) and some aphids, 



