is ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



that the third cleavage plane separates the ectoderm from the endoderm, the four 

 ventral cells being ectodermal, the four dorsal endodermal. Davidoff (1891 ) found 

 that in Distaplia the tour ventral cells are ectodermal, the four dorsal endodermal. 

 and a similar view is maintained by Samassa (1S'.>4). Castle (189G), on the other 

 hand, maintains that both the ventral and the dorsal cells of the 8-cell stage are 

 mixed, the ventral cells containing ectoderm and mesoderm, and the dorsal cells 

 endoderm and mesoderm, and not until the 48-eell stage are the substances of these 

 layers finally separated. 



My work, like that of Castle, places hut little weight upon the idea of germ 

 layers, since it undertakes to trace specific organs to certain cleavage cells, and 

 even to certain regions of the unsegmented egg. Emphasis is therefore placed upon 

 organs and upon organ-forming cells and substances rather than upon the more 

 indefinite germ layers. However, I find that the tour ventral cells of the 8-cell 

 stage are purely ectodermal, while the four dorsal cells are endodermal and meso- 

 dermal, save for the fact that four neural plate cells (A 74 , A 78 , figs. 120, 121, 123) 

 will arise from the anterior portion of the dorsal hemisphere at the 44-cell stage. 

 The mesoderm and endoderm are first completely separated at the 22-cell stage 

 (figs. 117, 118). I find that only four ectodermal (neural plate) cells come from the 

 dorsal hemisphere, whereas Van Beneden and Julin hold that at a corresponding 

 stage (44-cells), sixteen ectodermal cells have been derived from the dorsal hemi- 

 sphere. Of these sixteen cells four only are really ectodermal (the neural plate 

 cells), eight are mesodermal, and four are endodermal. Castle's conclusion that a 

 portion of the mesoderm is derived from the ventral cells is due to his erroneous 

 lineage of the cells after the 48-cell stage; all of the ventral cells are ectodermal, 

 and all of the mesoderm and endoderm are derived from the dorsal cells. With , 

 the exception therefore of these four neural plate cells, which arise at the 44-cell 

 stage on the dorsal side of the third cleavage plane, all of the ectoderm lies on the 

 ventral side of that plane, and all of the endoderm and mesoderm on its dorsal side. 

 This conclusion, it will he observed, is very similar to that of Seeliger, Davidoff,' 

 ami Samassa. 



4. Fourth Cleavage; 8-i6 cells. (Figs. 36-38, 110-115, 186-188.) 

 The spindles for the fourth cleavage appear in all of the eight cells at about 

 the same time, though the dorsal cells sometimes divide slightly in advance of the 

 ventral ones. All the spindles are approximately horizontal in position, and all are 

 oblique to the median and transverse planes (first and second cleavage planes). 

 As a result of the fact, stated on page 45, that the dorsal hemisphere is broad in front 

 and narrow behind, while the ventral hemisphere is broad behind and narrow in 

 front, we find that the obliquity of the spindles of one hemisphere is reversed as 

 compared with that of the other. Thus the spindles in the anterior-dorsal cells 

 approach in direction a transverse plane, in the posterior-dorsal cells they approach 

 an antero-posterior plane ; whereas in the anterior-ventral cells they approach an 

 autero-posterior plane, while in the posterior-ventral cells they approach a trans- 

 verse plane (figs. 110-113, 180, 187). 



