252 THE EMBRYOLOGY OF THE BLOW-FLY IN THE EGG. 



very large, and extend on either side of the yelk to the anterior 

 and posterior extremities of the embryo. They then lose their 

 connection with the alimentary canal, but remain open in front 

 of the intestinal flexure corresponding with the original blasto- 

 pore, and communicate with the segmentation cavity of Cholod- 

 kowsky. This cavity (/>, Fig. 39) is produced by the contraction of 

 the primitive band, and it communicates with the yelk in front 

 of the coelomic sacs and behind the procephalic lobes. As the 

 primitive band contracts, the head and tail-folds of the amnion 

 separate, so that the cavity, p, is only divided from the inter- 

 amnial space b}^ its roof of parablast. It becomes the pericardial 

 cavity. 



Segmentation of the Somatopleure and Coelomic Sacs. — Even 

 before the epiblast of the somatopleure covers the dorsal 



Fig. 41. — A dorsal view of an embryo before the somatopleural plates meet on the 

 dorsum of the yelk, showing their segmentation. 



surface of the yelk, the annular segmentation of the larva 

 becomes apparent by the formation of deep inflections between 

 the segments. These inflections at first only involve the 

 epiblast, but soon extend to the mesoblast. 



The coelomic sacs form two cellular plates, with a third 

 incomplete plate between them (PL XIV., Fig. 2). The external 

 and internal plates become adherent to the involutions of the 

 epiblast between the somites, so that each coelomic sac is 

 divided into a series of secondary cavities, corresponding 

 in number to the annuli of the epiblast. 



I have not hitherto seen this stage of development in any of 

 my sections, but Graber gives figures of embryos in which each 

 somite exhibits a distinct cavity between the outer and inner 

 coelomic plates. 



