BRACHIONUS. 



ovum divides into two unequal spheres, of which the smaller in the 

 subsequent stages segments more rapidly than the larger. The seg- 

 mentation ends with the formation of an epibolic gastrula. The solid 

 inner mass of cells derived from the larger sphere constitutes the 

 liypoblast, and is more granular than the epiblast. The evolution 

 of the embryo commences with the formation of a depression 011 

 the ventral surface, at the bottom of which the stomodaeum is formed 

 by an imagination. At the hinder part of the depression there rises 

 up a rounded protuberance which eventually becomes the caudal 

 appendage or foot. Immediately behind the mouth is formed an 

 imderlip. 



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 prseoral lobe, and the liypoblast is at the 

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

 ganglion is early formed as an epiblastic thickening on the dorsal 

 .side of the prseoral lobe. 



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



At a later period the lateral ridges 



t,- of the trochal disc meet dorsally 



and so enclose the preeoral 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 praaoral lobe. 

 Before this period the procto- 

 dseum is established as a shallow 

 pit immediately behind the in- 

 sertion of the foot. The latter 

 structure soon becomes pointed 

 and forked (fig. 100, t). 



The complete establishment of 

 the alimentary canal occurs late. 

 id. lateral The stomodffium (fig. 100) gives 



m 



<me 



Id 



FIG. 100. EMBRYO OF BRACHIONUS 

 URCEOLARIS SHORTLY BEFORE IT IS HATCHED. 



(After Salensky.) 



m. mouth; ms. masticatory apparatus; 

 me. mesenteron; an. anus; 



de ; 



and masticatory apparatus (ms). 



The mesenteron is formed from 

 the median part of the liypoblast ; the lateral parts of which appear 

 to give rise to the great lateral glandular structures (Id) which open 

 into the stomach, and to the ovaries (?) (pv) etc. The proctodaeum 

 becomes the cloaca and anus (an). The origin of the mesoblast is 

 not certainly known. The shell is formed before the larva is hatched 



