226 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



cells. Whether the division is amitotic or mitotic was not determined 

 by Roonwal. 



Yolk cells that do not share in the formation of the yolk-cell mem- 

 l)rane lie singly and are stellate; others group together into clumps of 

 two or three or more to form irregular syncytia. At about the fortieth 

 hour the yolk-cell syncytia may contain seven or even more nuclei in 

 each, but shortly afterward they disappear, and only single yolk cells 

 are met with. As time goes on, the yolk cells penetrate deeper into the 

 yolk and thus become distributed throughout the egg. 



In Locusta migratoria, according to Roonwal, there are four main 

 periods of activity into which the whole process of gastrulation is divided. 

 These periods are fairly sharply demarcated, although they sometimes 



am am.cav 



Fig. 144. — Locusta. Cross section of 46-hour embryo, (ajn) Amnion, (am. cav) Amniotic 

 cavity, (ect) Ectoderm, (il) Inner layer, (ser) Serosa. 



overlap. The first phase occurs by modified epiboly (overgrowth) and 

 results in the differentiation of the primary endoderm represented partl}^ 

 by the primary yolk cells. In the second phase the yolk-cell membrane 

 (corresponding to a part of the evanescent primary entoderm) is formed. 

 Multilayered condition of the germ band is an indication of activity 

 during this phase. In the third phase the second ventral groove (part of 

 the gastral groove) and the inner layer are formed. In the fourth phase 

 the secondary yolk cells (part of the secondary entoderm) appear. 



The embryonic envelopes, or membranes, in this species are formed 

 in the usual way and begin to appear almost simultaneously with the 

 differentiation of the inner layer. This is brought about by the inward 

 folding of the lateral borders of the germ band ventrally. The cephalic 

 fold appears first, then the caudal fold. At about the 50-hour stage the 

 (embryonic envelopes, amnion and serosa, are completed. Between the 

 amnion and serosa at the sides there is some yolk which may even con- 

 tain yolk cells, but in the middle the envelopes are nearly or quite in 

 contact with one another (Fig. 145). 



