48 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 аға the four ventral cells are ectodermal, the four dorsal endodermal, 
and a similar view is maintained by Samassa (1894). Castle (1896), 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-cell stage are the substances of these 
layers finally separated. 
My work, like that of Castle, places but 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 four 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 (A74, A*5, 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 Juli 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 be observed, is very similar to that of Seeliger, Davidoff, 
and Samassa. 
4. Fourth Cleavage; 8-16 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. АП the spindles are approximately horizontal in position, and all are 
oblique to the median and transverse planes (first and second cleavage planes). 
Ав 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 іп direction a transverse plane, in the posterior-dorsal cells they approach 
an antero-posterior plane; whereas in the anterior-ventral cells they approach an 
antero-posterior plane, while in the posterior-ventral cells they approach a trans- 
verse plane (figs. 110-113, 186, 187). 
