522 PROCEEDINGS OF THE ACADEMY OF [DeC, 



The third quartet consists of small cells from which apparently 

 only ectoderm is derived. The individual divisions of these cells ha-ve 

 not been traced very far, but there is every reason to believe that they 

 form ectoderm only. 



The history of the fourth quartet is peculiar. As already stated ^ 

 the posterior cell 4d is the mesentoblast, from which the alimentary 

 canal and a portion of the mesoderm arises. The other three cells of 

 the fourth quartet, 4a, 46 and 4c, do not divide as long as their history 

 can be traced. They, however, break up into a large number of 

 homogeneous yolk spheres which are absorbed by the endoderm cells.. 

 The large nuclei of these three cells can be traced until the alimentary 

 canal is partl}^ formed. 



The nuclei of the small macromeres show evidences of degeneration. 

 These do not divide as long as they can be followed. They are carried 

 into the embryo by the pharyngeal invagination of ectoderm, and it 

 seems probable that they degenerate without giving rise to any morpho- 

 logical structure. 



The Unsegmented Egg. 



The unsegmented egg of Planocera inquilina is nearly spherical in 

 shape and measures about one-tenth of a millimeter in diameter. In 

 many cases, however, the eggs are pressed out of their normal shape 

 by crowding within the capsule. The eggs when laid possess a large 

 germinal vesicle wdiich lies slightly to one side of the centre (fig. 1). 

 This statement is not remarkable in itself, were it not for the fact that 

 the eggs in the uterus of the adult possess a well-developed spindle 

 with equatorial plate, centrosomes, etc. Apparently this spindle 

 never goes farther than the equatorial plate stage and then degenerates 

 so that the egg when deposited possesses a germinal vesicle. This 

 phenomenon was first observed in certain polyclad eggs by Selenka 

 (81d), and later Wheeler (94) has recorded it for this species. Gardiner 

 (99) has studied this phenomenon in Polycherus candaius and concludes 

 that the uterine spindle is due to abnormal conditions of the adult. I 

 have not attempted to study this phenomenon in detail, but a casual 

 survey shows that in animals which were fixed as soon as possible after 

 removal from the whelk this uterine spindle was well developetl. 

 Since other animals from these same lots laid eggs which developed 

 normally, one must conclude that if it is not a normal phenomenon it 

 at least does not interfere with the later development. Selenka (81f/) 

 suggests that this spindle is of use in bringing the yolk granules to 

 the centre of the egg, but, as Wheeler has noted, such could hardly be 



