467 



has, scattered through it, very large and heavily-staining nuclei. One 

 of these nuclei is shown in the figure situated at the anterior end and 

 another at the posterior end, in very noticeable thickenings, of the 

 layer. The nuclei of the serosa have increased greatly in size and 

 are now very conspicuous, lying just beneath the vitelline membrane. 



The serosa is the only embryonic membrane that develops in the 

 case of Camponotus to enclose the embryo entirely. This develops, 

 as has been seen, by a backward growth as a single layer from the 

 anterior border of the cephalic groove. At the posterior end of the 

 germ band it does not unite with the posterior edge of the caudal 

 groove, but grows on backward to enclose the rest of the blastoderm. 

 This differs from the typical method of formation of embryonic 

 membranes in two ways: (i) the entire membrane is formed by a 

 growth from the cephalic fold, the caudal fold being rudimentary; 

 (2) the growth occurs as a single and not as a double layer. There 

 is, therefore, no amnion formed over the ventral surface of the 

 embryo. The dorsal closure, however, has been effected, as has been 

 described, by a delicate one-celled layer which is continuous with 

 the edges of the germ band. This layer is similar in structure to 

 the serosa. It grows out from the edges of the germ band, spreads 

 dorsally, and closes over to form the dorsal body-wall of the em- 

 bryo; hence it is to be regarded as the amnion. 



Graber has shown ('88, pp. 144-146) that there are two em- 

 bryonic membranes in Polistes gallica, Formica rufa, and Hylotoma 

 berberidis. He says that the inner layer becomes closely applied to 

 the germ band and is indistinguishable from the latter. In Campono- 

 tus I have been unable to find more than the one layer on the ventral 

 side of the embryo. Carriere ('97, pp. 396) found but the one layer 

 in Polistes gallica and Chalicodoma miiraria, and Biitschli (1870) 

 found but one in Apis. Ganin, who studied the development of sev- 

 eral species of Formica and Myrmica, also says that there is but one 

 embryonic layer. 



The ventral part of the embryo, which is in the position of the 

 original germ band, is now much narrower than the original germ- 

 band and is in the form of a narrow ridge-like thickening along the 

 median ventral line, widening out at the anterior, and, to a less ex- 

 tent, at the posterior end. Figure 18, Plate V, represents a cross- 

 section of this stage showing the ridge-like thickening on the ventral 

 side, and the large cells forming a layer extending about half-way 

 around on the ventral side. The anterior widening shows the funda- 

 ments of appendages and of the stomodaeal invagination, but the 

 proctodaeal invagination does not appear until several days later. 



