ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 65 



Tlu' second factor <>f the gastrulation, viz., the overgrowth of the cells sur- 

 rounding the endodermal area has been well described by Castle. It is the result 

 in part of the first factor and also of the more rapid division of the cells of the ecto- 

 derm and the corresponding retardation of division in the endodermal cells. This 

 overgrowth occurs in the anterior quadrants from in front, the chorda cells over- 

 growing the endoderni and the neural plate cells the chorda : in the posterior quad- 

 rants it occurs from the sides, the muscle cells overgrowing the mesenchyme, and 

 the ectoderm the muscle cells. At three points this overgrowth is long delayed, at 

 the posterior pole where there is a dee]) notch In the blastoporic rim which persists 

 until the blastopore has nearly closed, and at the light and left sides of the blasto- 

 pore where the overgrowth is slow. This leads to the peculiar form of blastopore, 

 wide in front and narrow behind, which is found among ascidians. 



8. Eighth Cleavage; 1 12-132 cells, 132-218 cells. (Figs. 135-147, 205). 



The eighth cleavage first appears in the two anterior muscle cells of each side 

 ( B 88 , B 87 ), the spindles being nearly transverse to the antero-posterior axis of the 

 embryo (figs. 135, 136). This division is equal and non-differential, and there result 

 four muscle cells on each side, an anterior pair (B 915 , B 916 ) and a posterior pair (B 913 , 

 B 914 ). When first formed the median cells of each of these pairs lie at a higher 

 level than the lateral ones (fig. 135) ; soon afterward the lateral and median cells 

 are at the same level (fig. 136); still later the lateral ones lie at a higher level 

 than the median ones (fig. 140). This is, of course, a result of the overgrowth, 

 whereby the cells which were lateral in the blastopore lip come to overly those 

 which were median in position. 



At the same time that these muscle cells are dividing the pair of large mesen- 

 chyme cells, B 85 , divides, the spindles being obliquely antero-posterior and dorso- 

 ventral in direction (figs. 135, 136). This division is approximately equal and 

 non-differential, and gives rise to the mesenchyme cells B", .B 910 , which lie on each 

 side of the caudal endoderm cells. 



While these divisions are proceeding in the mesoderm, and thereby advancing 

 these cells to the ninth generation, the delayed seventh cleavage appears in the 

 mesenchyme and endoderm cells. The first of these cells to divide is the most 

 anterior mesenchyme cell (A 7 - 6 ) ; the- spindles are here nearly dorso-ventral in 

 direction, and the resulting daughter cells (A su , A 8 - 12 ) are of about the same size, 

 though the dorsal cell contains more protoplasm than the ventral one, as Castle has 

 shown. 



Coincidently with these divisions two pairs of endoderm cells (A 72 and B 72 ) 

 divide, the spindles being approximately transverse in the anterior pair and antero- 

 posterior in the'posterior one (fig. 135). 



A little later the four endoderm cells which meet at the vegetal pole (A 71 , B 71 , fig. 

 1 36) divide, the spindles being antero-posterior in direction. The last remaining pair 

 of endoderm cells of the seventh generation to divide is the lateral one (A 75 , figs. 135, 

 136) ; I have not seen this cell in division, but it is probable from a study of later 



9 JOUEK. A. N. S. PHI I, A., VOL. XIII. 



