26 CONK LIN. [Vol. XIII. 



The first cleavage is transverse to the long axis of the embryo, 

 exactly as it is in the case of Teredo, Nereis, and Umbrella, 

 and divides the ovum into an anterior and a posterior half ; 

 the second cleavage coincides with the antero-posterior axis of 

 the future embryo, and divides the ovum into right and left 

 moieties. The four macromeres formed by the first two cleav- 

 ages are nearly equal in size, and each contains the elements 

 of both ectoblast and entoblast, and the left posterior macromere 

 contains, in addition, most of the future mesoblast. 



For-mation of the Ectoblast. — The whole of the ectoblast is 

 separated from the macromeres by three successive divisions, 

 which separate twelve micromeres from the four macromeres. 

 The four cells first separated from the macromeres constitute 

 the first quartette of micromeres, while those separated by the 

 two following divisions are respectively the second and third 

 quartettes. The first quartette forms the upper hemisphere 

 (umbrella or head vesicle) of the larva, the brain, an apical 

 sense organ, an apical plate of ciliated cells, and a portion of 

 the velum. The second quartette gives rise to the larger part 

 of the velum, the shell gland, and at least a part of the foot. 

 The third quartette I have not been able to follow satisfac- 

 torily ; its derivatives lie wholly outside of the velar area, and 

 form a considerable part of the lower hemisphere. 



Formation of Mesoblast and Entoblast. — Soon after the 

 ectoblast has been segregated, and at the stage when there 

 are twenty-four cells, the left posterior macromere divides 

 obliquely, forming the first member of the fourth quartette, 

 which later comes to lie in the second cleavage furrow at the 

 posterior side of the ^gg. This cell then divides into right and 

 left portions, and each half again divides into a dorsal and 

 ventral part. The two ventral moieties form a part of the 

 intestine or hinder portion of the alimentary canal. The two 

 dorsal moieties are still mesentoblasts, and the mesoblast is not 

 completely separated from the entoblast until after two more 

 divisions. There are finally formed two mesoblastic teloblasts, 

 each of which gives rise to a mesoblastic band, from which a 

 part of the middle layer is derived. The rest of the middle 

 layer comes apparently from one additional mesoblast cell in 



