34 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



designated the "blastema" by Weismann. When the definitive number 

 of peripheral nuclei have been formed, cell walls appear, first between the 

 nuclei and then on the side toward the yolk, thus completing the cells and 

 with them the blastoderm. Some writers who regard the yolk cells as 

 the primary entoderm substitute for the term "blastoderm" as used here 

 the term "primary epithelium" on the ground that an entoderm being 

 already present, the stage itself is really a gastrula. In the formation of 

 the blastoderm in Calliphora, Diacrisia, and many other insects the 

 cells at the time they reach the periplasm are closely spaced; in other 

 forms, as in Carausius, they are far apart except those in one area that 

 forms the small embryonic rudiment. In the stone fly Pteronarcys and 

 in the centipede Scolopendra the blastoderm arises from isolated cell 

 groups which have developed from the outward-migrating cells. The 

 examples given here represent extremes between which intermediate 

 types may be found in several orders. 



YOLK CELLS 



In the outward migration of the cleavage cells toward the periphery 

 some remain behind in the yolk and form the so-called "vitellophags," 

 or primary 3^olk cells. This intra vitelline separation, as it was termed by 

 Heymons, occurs in Machilis, Lepisma, Heteroptera, Homoptera, Lepi- 

 doptera, Coleoptera, Diptera, most Hymenoptera, and some Orthoptera 

 (Locusta, Gryllus, Oecanthus, Gryllotalpa) . With some other insects all 

 cleavage cells reach the periphery, but some to return later into the yolk 

 as secondary yolk cells. This may occur either by a centripetal migration 

 of cells from the peripheral layer or by a division of a peripheral cell. 

 The centripetal migration has been recorded in Campodea, Pteronarcys, 

 Neophylax, and several genera of Orthoptera (Periplaneta, Blattella, 

 Gryllotalpa, Mantis, Carausius). In some cases it seems that both types 

 may occur as in Gryllotalpa among Orthoptera, Calliphora and Melo- 

 phagus among Diptera, and Brachyrhinus (Butt, 1936) among Coleoptera. 

 Although numerous writers record amitotic (direct) division of yolk cells, 

 mitotic division has been observed in Pieris (Eastham, 1927), Ephestia 

 (Sehl, 1931), Calandra (Tiegs and Murray, 1938), Apis (Dickel, Nelson, 

 1915), and some others. It is probable that all j^olk cells may undergo 

 amitosis during senescence. 



The function of the yolk cells is to liquefy the yolk, rendering it 

 readily assimilable by the embryo; hence the names "vitellophags" and 

 "trophonuclei" are also applied to them. Uichanco (1924) calls them 

 " mycetoblasts " because of the part that they play in the development of 

 the mycetom in aphids. Toward the close of embryonic life they 

 disintegrate in the yolk, only exceptionally being present in post- 

 embryonic life. Because of their early appearance they have been 



