350 CLEPSINE. 



of six cells, and on the opposite side a layer formed of four cells. The 

 two layers meet at the front border of the embryo and between them are 

 placed the three large vitelline spheres. The two patches of epiblast cells 

 now rapidly increase, and gradually spread over the three large vitelline 

 spheres. Except where they meet each other at the front edge they leave 

 uncovered a large part of the margin of the vitelline spheres. 



While these changes have been taking place on the exterior, the 

 hypoblast cells have increased in number (additional cells being probably 

 derived from the three large vitelline spheres) and fill up in a column-like 

 fashion a space which is bounded behind by the three vitelline spheres, and 

 in front by the epiblast of the anterior end of the embryo. At the sides of 

 the hypoblast the mesoblast has become established, probably as two lateral 

 bands. The origin of the cells forming it has not yet been determined. 

 The hypoblast cells in the succeeding stage arrange themselves round a 

 central archenteric cavity, and at the same time rapidly increase in size 

 and become filled with a secondary deposit of food-yolk. Shortly after- 

 wards a mouth and thick-walled oesophagus are formed, probably from an 

 epiblastic invagination. The mesoblast now forms two curved lateral 

 bands at the two sides of the body, equivalent to the mesoblastic bands 

 of the Chaetopoda. The three vitelline spheres, still largely uncovered by 

 the epiblast, lie at the posterior end of the body. The embryo grows 

 rapidly, especially anteriorly, and the three vitelline spheres become 

 covered by a layer of flattened epiblast cells. Around the oesophagus a 

 cavity traversed by muscular fibres is established. Elsewhere there is no 

 trace of such a cavity. The cephalic region becomes ciliated, and the 

 'dorsal part of it, which represents a rudimentary prje-oral lobe, is especially 

 prominent. The cilia of the oral region are continued into the lumen of 

 the oesophagus, and at a later period are prolonged, as in Lumbricus, along 

 the median line of the ventral surface. 



The mesoblastic bands would seem from Biitschli's observations, which 

 receive confirmation from Kleinenberg's researches on Lumbricus, to be pro- 

 longed dorsally to the oesophagus into the cephalic region. Posteriorly they 

 abut on the large vitelline spheres, which were supposed by Kowalevsky 

 to give origin to them, and to play the same part as the large meso- 

 blasts in Lumbricus. It has already been shewn that the function of the 

 large cells in Lumbricus has been exaggerated, and Biitschli denies to 

 them in Nephelis any share in the production of the mesoblast. It seems 

 in fact probable that they are homologous with the three vitelline spheres 

 of Clepsine ; and that their primitive function is to give origin to the 

 hypoblast. They are visible for a long time at the hind end of the embryo, 

 but eventually break up into smaller cells, the fate of which is unknown. 



The embryo of Hirudo would appear from the researches of Robin 

 to develop in nearly the same way as that of Nephelis. The anterior 

 part is not however ciliated. The three large posterior cells disappear 

 relatively early. 



