M. ТАК Meee eee ee eee loe 
ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 61 
sal side, are directed obliquely forward, outward and ventralward (figs. 131, 189). 
These divisions, when completed, give rise in the anterior quadrants to four chorda 
cells (A55, AM, А%Ч% AS! and to four neural plate cells (А%7, A®*, Д®һ A*!5) on 
each side of the mid-line, which are arranged in two rows of eight cells each 
running around the anterior border of the dorsal hemisphere (figs. 134, 200). All 
these divisions are equal and non-differential. 
In the posterior quadrants the division of the anterior muscle cell on each side 
gives rise to two daughter cells (B^, D55), one in front of the other, which are alike 
in size and quality. The mesenchyme cell (B??) divides unequally and differentially 
giving rise to a small yolk-laden cell (B*5), lying anterior, lateral and ventral to its 
large sister cell (B**), which is more protoplasmic (figs. 134, 200). Тһе former is, 
according to Castle, the “ posterior chorda fundament” while the latter is mesen- 
chyme. I have been unable to find sufficient evidence that this small cell is later 
incorporated in the chorda, but on the other hand do not wish to deny that this is 
the case. Since it is derived from the mesenchyme I prefer to class it with the 
mesenchyme cells until its fate is more certainly known. These divisions advance 
the embryo to the 76-cell stage and the distribution and generations of the cells 
may be summarized as follows : 
Ventral hemisphere 
POLES eie a oan etri И ith generation 26 cells. 
TERME SIME sus QUERI ues Tth " B 
Dorsal hemisphere | 
ЖЕНИС, е, кы ииз 7th 5 10726 
Chorda. . . . 8th generation 8 cells. 
Neural plate 8th e 
Muscle... . 8th s E UH d AU 
Mesenchyme 8th > ыс TEB ы; B 
Sth generation 94 cells. Tth ener 52 cells. 
76 cells. 
The classification of the cells of this stage into chorda, neural plate, muscle 
and mesenchyme," must of course be based upon the later history of these cells, but 
even at this early stage important differences may be recognized in the histological 
characters of the cells named. In the living eggs of CyzzZza the endoderm cells 
are slate-gray in color and are filled with yolk; the chorda cells are lighter 
gray and contain less yolk than the endoderm cells; the ectoderm and neural 
plate cells are clear and protoplasmie; while the muscle and mesenchyme cells 
are yellow, the forme* being more deeply colored than the latter. In the main 
my classification. of the cells of this stage agrees with that of Castle; the most | 
ac concerns the muscle cells which he classes as са neuro-muscular 
