222 



BRACHIONUS. 



female ovum divides into two unequal spheres, of which the 

 smaller in the subsequent stages segments more rapidly than the 

 larger. The segmentation ends with the formation of an epibolic 

 gastrula. The solid inner mass of cells derived from the larger 

 sphere constitutes the hypoblast, and is more granular than the 

 epiblast The evolution of the embryo commences with the 

 formation of a depression on the ventral surface, at the bottom 

 of which the stomodaeum is formed by an invagination. At the 

 hinder part of the depression there rises up a rounded protuber- 

 ance which eventually becomes the caudal appendage or foot. 

 Immediately behind the mouth is formed an underlip. 



On the sides of the ventral depression are two ridges which 

 form the lateral boundaries of the trochal disc. They appear to 

 unite with the under lip. 



In a later stage the anterior part of the body becomes marked 

 off from the posterior as a praeoral lobe, and the hypoblast is at 

 the same time confined to the posterior part. The supra-ceso- 

 phageal ganglion is early formed as an epiblastic thickening on 

 the dorsal side of the praeoral lobe. 



The first cilia to appear arise at the apex of the praeoral lobe. 

 At a later period the lateral 

 ridges of the trochal disc meet 

 dorsally and so enclose the prae- 

 oral lobe. They then become 

 coated by a ring of cilia, to which 

 a second ring, completing the 

 double ring of the adult, is added 

 later. 



In the trunk an indication of 

 a division into two segments 

 makes its appearance shortly 

 after the development of the 

 praeoral lobe. Before this period 

 the proctodaeum is established as 

 a shallow pit immediately behind 

 the insertion of the foot. The 

 latter structure soon becomes 

 pointed and forked (fig. 100, /). 



The complete establishment 



ov 



FIG. 100. EMBRYO OF BRACHIO- 

 NUS URCEOLARIS SHORTLY BEFORE IT 

 is HATCHED. (After Salensky.) 



vi. mouth ; ms. masticatory appa- 

 ratus; me. mesenteron ; an. anus; Id. 

 lateral gland ; ov. ovary ; t. tail, i. e. 

 foot ; tr. trochal disc ; sg. supra-ceso- 

 phageal ganglion. 



