35-i PROCEEDINGS OF THE ACADEMY OF [J^^lly, 



relative sizes of the macromeres in these two genera. Some of the 

 individual features in which the cleavage of Fulgur resembles that of 

 Crepidula are the following : 



a. The direction, rh3^thm and quality of each cleavage is the same. 



b. The first and second cleavages are equal; the first is transverse 

 to the median plane of the embryo, the second coincides with that 

 plane; a small yolk lobe is formed during each of these cleavages. 



c. The ectoderm is segregated in three quartets of micromeres. 



d. The first formed member of the fourth quartet, M, is the mesento- 

 mere; its anterior portion gives rise to the mesodermal bands and its 

 posterior portion to the intestine. 



e. The remaining members of the fourth quartet, viz., 4 A, 45, 4C 

 are entomeres and give rise to all portions of the alimentary canal and 

 its outgrowths, save the intestine and stomoda3um. 



/. The first quartet of ectomeres forms a "cross", wdth its centre at 

 the apical pole and with anterior, posterior, right and left arms. Be- 

 tween the arms are the "turret" cells, and between these and the apical 

 pole are the "rosette" cells; all of these cells are derived from the first 

 quartet, except the "terminal" cells of the arms which are derived 

 from the second quartet. The cross is formed of exactly the same cells 

 in Fidgur as in Crepidula, but it is neither so distinct in form iwr so 

 persistent. 



g. The second and third quartets surround the first and are com- 

 posed of cells which divide in the same order and direction as in Crepi- 

 dula. 



3. In later cleavages many more ectoderm cells are formed in Fulgur 

 than in Crepidida at corresponding stages of differentiation. 



4. The overgrowth of the yolk is highly peculiar in Fulgur. By 

 very great extension of the anterior half of the blastoderm, while the 

 posterior half remains relatively fixed, all the organ bases are carried 

 to the posterior margin of the blastoderm, where they form a kind of 

 germ ring. Subsequently the posterior margin also moves over the 

 yolk, so that the blastopore is finally formed at the vegetal pole. 



5. Before the extension of the anterior portion of the blastoderm 

 an apical invagination of ectoderm cells is formed in the region of the 

 anterior arm of the cross. This invagination subsequently flattens 

 out and completely disappears. Its function seems to be connected 

 with the rapid extension of this part of the blastoderm. 



6. The cerebral ganglia arise on each side of the apical invagination, 

 probably from the anterior "rosette" cells. The velar cells arise 

 around the outer margins of the ganglia, probably from the anterior 



