ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 71 



rior li]> these rows are tilted up into a dorso-ventral direction. An optical section 

 of the caudal region at this stat:c shows four muscle cells on each side, one above 

 another (fig. 158) ; a lateral view shows three rows of muscle cells with four or five 

 cells in each row (figs. 56, 157). I have not observed the exact manner in which 

 this change from two rows to three takes place, but it is evident that it must be 

 associated with the division of the cells of the original two rows. In figure 51 

 there are eight muscle cells on each side; in figure 55. thirteen ; in figure 56, 

 fourteen; in figure 157. fifteen ; in figure 165, eighteen; and in figure 59, twenty; 

 therefore each of the cells shown in figure 51 must have divided once and some 

 of them twice during the period represented by these figures. 



2. Development of Larva (Figs. 57-60, 160-167). 



After the growth of the anterior lip has carried the notochord to a posi- 

 tion approximately corresponding to that of the blastopore groove in figure 153 

 and has shifted the rows of muscle cells into a nearly vertical direction (figs. 

 157. 158), these rows of muscle cells again come to be antero-posterior in direc- 

 tion (figs. 58, 59, 161-165). This change takes place rather suddenly and I have 

 not observed all the steps in the process. It seems probable, however, that 

 it is due to two factors; (a) the depression of the dorsal ends of the muscle 

 rows to a position alongside of the notochord, and (b) the outgrowth of the 

 tail of the larva from the region of the ventral ends of the muscle rows. This 

 outgrowth, which is associated with the lengthening of the ventral side of the 

 embryo, carries the caudal mesenchyme cells and the ventral ends of the muscle 

 rows backward into the tail and thus the rows of muscle cells again assume an 

 anteroposterior direction (figs. 58, 59, 161-167). Usually six cells are seen in 

 each row and in addition there are two or more cells at the hinder ends of 

 these rows which do not fall specifically into any one of them. In living- 

 embryos the muscle and mesenchyme cells retain their yellow color and the 

 individual muscle cells may be plainly seen ; all the figures shown in plate V 

 represent camera drawings of living embryos and in all of them the yellow cells 

 were distinctly visible as drawn. 1 When seen from the caudal pole (fig. 60), the 

 three rows of muscle cells are seen to be only one-layered and the cells of one side 

 are connected with those of the other by a group of small yellow cells (the caudal 

 mesenchyme), "which lie ventral to the notochord at its hinder end. 



In stained preparations of young tadpoles these caudal mesenchyme cells can 

 be seen to consist of two or three pairs of cells at the posterior end of the caudal 

 endoderm and ventral to the notochord (figs. 101-165). The other mesenchyme 

 cells, which in a former stage (figs. 156, 157) were continuous with this caudal 

 group, are now separated from it by the whole length of the muscle rows. These 

 mesenchyme cells at the anterior ends of the muscle rows are found in later stages 



' Sine.' this paper was sent to press Misses I'd. it and Strobell have prepared for me a series of 

 more than thirty photomicrographs of the living eggs and embryos of Cynthia. These photographs 

 show in the most striking manner the yellow protoplasm and the cells which arise from it ; even in the 

 tadpole stage these individual cells are plainly recognizable in the photographs. 



