EARLY DEVELOPMENT 39 



where. When the time for differentiation of the embryo is at hand, all 

 these elements presumably move to the center of the yolk and stream 

 down en masse to the middle of the venter, where they pile up to form a 

 more or less oval multinucleate mass which is the material of the embry- 

 onic rudiment. 



Eggs of insects in many cases are more or less elongated, sometimes 

 convex on one side and straight or even concave on the other. In such 

 cases the embryo usually forms on the convex side. This, however, is by 

 no means a criterion, since in Hydrous, as Heider has shown, eggs are as 

 often curved ventrally as dorsally and in some instances even laterally. 

 Where the germ band forms at the end rather than the side, it is almost 

 always near the posterior pole. 



SEGMENTATION 



Superficial segmentation of the germ band in insects begins about the 

 time that the germ layers develop. Metamerism is apparent in the 

 transversely divided mesoderm and usually later in the transverse grooves 

 formed in the ectoderm. In some cases the ectoderm exhibits segmen- 

 tation before the mesoderm, as pointed out by Eastham (1927) in the 

 butterfl}^ Pieris. Soon after forming, the germ band shows a broader 

 head region and a narrow posterior portion; the former is the proto- 

 cephalon, the region of the procephalic lobes which will bear the labrum, 

 the mouth, the eyes, the antennae, and the rudimentary post antennae 

 (Figs. 28, pro.c, 29). The posterior part, or protocorm (p^c), is the portion 

 that wall give rise to the gnathal (jaw) segments of the head, the thorax, 

 and the abdomen. In most insects the protocorm divides immediately 

 into its definitive segments beginning with the anterior end. Division 

 of the abdomen is retarded in some instances, as in parasitic Hymenoptera, 

 until postembryonic hfe. In Brachyrhinus, Stenobothrus, etc., segmenta- 

 tion begins in the thorax and proceeds both forward and backward. 



Less frequently the protocorm is first divided into a few major 

 divisions, or macromeres, which later subdivide into the definitive seg- 

 ments. In Locusta migratoria, Roonwal (1936) describes the germ band 

 as first dividing into a protocephalic and three protocormic elements, 

 wherein the inner layer has undergone a segmentation into four parts 

 roughly corresponding to the external primary segmentation. Hirschler 

 (1909) has described a similar segmentation in the chrysomelid beetle 

 Donacia. Early segmentation of germ band in Isotoma, according to 

 Philiptschenko, occurs by the formation of the head lobes, the three 

 gnathal segments, and the first thoracic segment, the remainder of the 

 body not becoming segmented until later. Other examples of primary 

 segmentation have been described for Oecanthus, Stenobothrus, Melasoma, 

 Meloe, and the Platygastrinae. In Xiphidium Wheeler (1890) found that 



