458 HOLMES. 



EXPLANATION OF PLATE XXL 



Fig. 49. Mesoblastic pole cells, showing the small cells budded off at the 

 anterior end. 



Fig. 50. Optical section of an egg of about the stage shown in Fig. 41. 

 P.M., primary mesoblast ; S.M., secondary mesoblast. 



Fig. 51. Posterior side of a young embryo, showing the large head vesicle, 

 h.v., and the beginning of the shell gland, s.g. 



Fig. 52. Optical section of a slightly later stage. M., mouth ; s.g., shell gland ; 

 e.g., rudiment of cerebral ganglion which is proliferating cells ; /., prototroch ; e.c., 

 entodermic cells that have become gorged with albuminous matter; mes., cells of 

 the mesoblastic band ; g.c., giant cell that has become partially perforated. 



Fig. 53. Surface view of the same stage, i^., mouth; /., prototroch ; e.g., 

 rudiment of cerebral ganglion. The patch of clear cells around the mouth gradu- 

 ally narrows posteriorly until it becomes a narrow band of clear cells separating 

 the two halves of the foot. 



