550 proceedings of the academy of [dec, 



Summary. 



The cleavage of the eggs of Plcmocera inquilina Wh. until a late stage 

 (forty-four cells) is strictly spiral in the dextral sequence. 



Three quartets of ectomeres are given off in alternating dexiotropic 

 and Iseotropic directions. At the next division a fourth quartet is 

 formed, the cells of which are of very large size and contain most of 

 the yolk. The "macromeres" are very minute cells which remain at 

 the vegetative pole until the closure of the blastopore. The marked 

 degenerative character of their nuclei and the small amount of cyto- 

 plasm indicate that they degenerate without giving rise to any struc- 

 ture (p. 529). 



At the stage with forty cells there are formed at the animal pole 

 four small "apical" cells, which in their method and time of origin 

 correspond closely to the cells of the same name in annelids and 

 mollusks (p. 530). 



At the forty-four-cell stage the posterior cell of the fourth quartet, 

 M, buds a single large cell into the interior of the embryo. Both of 

 these cells, 4d^ and 4:(P, next divide bilaterally (p. 536). 



Of these four cells the two upper and inner give rise to a portion of 

 the mesoderm and possibly a small part of the endoderm (p. 537). The 

 lower pair of cells, lying on the surface of the embryo, give rise to prac- 

 tically all of the endodermal part of the alimentary canal (p. 541). 

 Thus the history of this cell, 4:d, shows a remarkable resemblance to 

 its homologue in mollusks and annelids. 



The three anterior cells of the fourth quartet, 4a, 46 and 4c, seem to 

 function only as the bearers of food yolk and apparently give rise to 

 no morphological structure. The very large nuclei of these cells can 

 be followed until the beginning of the pharyngeal invagination. The 

 yolk in these cells breaks up into spherules, probably through the 

 action of enzymes from the large nuclei. This liquified yolk is later 

 absorbed by the endoderm cells (p. 542). 



A large portion of the ectoderm is formed by the successive budding 

 or delimination of small cells from larger, deeper lying ones (p. 533). 



A portion of the mesoderm, chiefly that part lying around the 

 pharynx, is derived from cells of the second quartet, and thus corres- 

 ponds with the "secondary" mesoblast or "larval" mesenchyme of 

 annelids and mollusks (p. 535). 



In the spiral cleavage, the segregation of the ectoblast in three 

 quartets, the formation of a large part of the mesoderm from 4(i, the 

 formation of the apical cells, and in many other details the development 



