38 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



blastoderm formation; with the determinative type it occurs before 

 blastoderm formation. 



The differentiation of sex cells from the soma in general represents 

 one of the first of the differentiations of organs to take place in the germ 

 band, even though in some cases the germ cells appear later than in others. 



GERM BAND 



The length of the germ band, or germ disk, either when it first appears 

 or when it has reached its greatest length as an embryo, is not correlated 

 with the length of the insect at the time of emergence. In a comparative 

 study the relation of length to breadth of the embryo should be con- 

 sidered with reference to the form of the egg. In general, it may be said 

 that the form of the egg has but little relation to the form or length of 

 either the early or the late germ band. Graber (1890) attempted a 

 classification of the types of germ bands on the basis of maximum length 

 and degree of flexure, but it will be seen that only within rather narrow 

 limits is it of phylogenetic significance. He recognized three types: (1) 

 long (tanyblastic) and flexed (ankyloblastic), (2) short (brachyblastic) 

 and straight (orthoblastic), and (3) intermediate. In the first group he 

 placed some Coleoptera (Lina, Donacia, Lema, Telephorus), Lepidoptera, 

 Phryganidae, and some Hemiptera; in the second group, Libellulidae and 

 some Orthoptera {Blatta, Stenohothrus, Mantis, Oecanthus); and in the 

 third group some Coleoptera (Hydrophilus, Melolontha), some Orthoptera 

 (Gryllotalpa) , and Hymenoptera {Apis, Formica, Hylotoma). Of other 

 orders not mentioned by Graber the Diptera and Collembola would 

 find a place in the first group to which the Myriapoda may be added. 

 The maximum length of the embryo bears but little relation to its initial 

 length, however, as may be seen in comparing the minute disk-like early 

 rudiment of the walking stick Carausius with its definitive form. 



In some insects the blastoderm consists of a layer of deep cells uni- 

 formly distributed over the entire surface after which there is a thinning 

 out of the nonembryonic part, as in the Mallophaga (Strindberg, 1916), 

 Apis (Nelson, 1915), the muscids (Graber, 1889). In other insects the 

 cells at the time of formation of the blastoderm are already differentiated 

 into germ band and extraembryonic tissue, as in Diacrisia (Johannsen, 

 1929) and Euvanessa (Woodworth, 1889). The germ-band formation in 

 Pteronarcys as described by Miller (1939) is unusual. Here compound 

 "primary nuclear aggregates," which seem to represent the prospective 

 embryonic cells, appear after the time of the outw^ard migration of the 

 cleavage cells; they presumably arise by clumping of cells that migrate 

 inward from the periphery. The embryonic rudiment is formed evidently 

 from the nuclear aggregates in the yolk and by individual plump cells 

 scattered near or in the ventral primary epithelium and perhaps else- 



