,8 WHEELER. [Vol. VIII. 



groove extending nearly the entire length of the germ-band 

 and dividing it into two symmetrical halves. Anteriorly the 

 groove is rounded and seems to end rather abruptly, but 

 posteriorly it bifurcates, each of the two grooves thus arising 

 beino- continued for a short distance to either side till they 

 gradually fade away. There can be no doubt, it seems to me, 

 that the bifurcated termination of the blastopore is the homo- 

 logue of the similar structure first figured by me in Doryphora 

 (•89, PI. XVIII, Fig. 71; PI. XIX, Fig. 73) and subsequently 

 seen by Graber ('90) in the corresponding stages of Lina 

 tremulcB (PI. II, Figs. 25 and 27). More recently Cholodkowsky 

 has observed a similar widening of the blastopore in Blatta. 

 He attempts to identify it with the posterior depressions of 

 Graber's "lateral gastrulation." 



In Stage B (Fig. 2) when the caudal amnio-serosal fold has 

 covered the ligulate portion of the germ-band, the blasto- 

 pore presents a widening of its anterior end at a point which 

 probably lies just in front of the definitive mouth. This widen- 

 ing was observed in several embryos, and I therefore take it 

 to be a normal occurrence. It also has its homologue in the 

 Doryphora embryo (see my Fig. 70, PI. XVIII, '89). In the 

 stage under consideration (Fig. 2) the anal bifurcation has 

 grown more indistinct and is apparently about to disappear. 



The closure of the blastopore proceeds simultaneously in 

 two directions : from its anterior end backwards, and from its 

 posterior end forwards, so that the last portion of the groove 

 to disappear lies in that part of the germ-band which is to 

 become the thoracic or baso-abdominal region. 



In sections the groove is seen to be much shallower than it 

 appears in surface view. Along its whole extent its floor is 

 somewhat thickened and in this portion — destined to form the 

 mesentoderm — the cells soon lose their columnar shapes and 

 become more polygonal in outline and more irregular in their 

 arrangement. The groove closes in such a way that no 

 tubular cavity results as in the Coleoptera and Diptera; the 

 cells at the edges of the depression appear to glide over the 

 median elements, so that after the fusion of .the edges in the 

 median line the mesentoderm has the form of a solid cord 



