150 Ediuin G. Conkhn. 



to occupy six cells (B*"' ^ B^-^, B*'-^) around the posterior border (the 

 spindles which lead to the formation of these six cells are indicated 

 in Fig. 4). These thirty-two cells are increased to sixty-four at 

 the next cleavage (Figs. 5 and 6); during this cleavage four chorda 

 cells (A^-^ A^-^) are separated from the four neural plate cells 

 (A^'', A^-^, Fig. 6), while the six cells of the yellow crescent have 

 given rise to twelve, four of which are muscle cells (B^ % B^-^) and 

 eight mesenchyme (B^^ B^ ^ B^% B^"). At the same time an 

 additional pair of mesenchyme cells (A^'') is separated from a 

 pair of endoderm cells in the anterior quadrants. This is the only 

 mesenchyme cell derived from the anterior quadrants. 



At this stage all the substances of the germ layers and of the 

 principal organs of the larva are gathered into separate cells, but 

 although this segregation into separate cells comes relatively late 

 in the cleavage these substances have been definitely localized in 

 certain regions of the egg from the time of the first cleavage. 

 Subsequent cleavages lead to changes in the shape of the embryo 

 but produce no changes in this localization. 



In the gastrulation the endoderm cells are depressed and are 

 overgrown in front by the chorda cells and these in turn are 

 covered by the neural plate cells; similarly the mesenchyme cells 

 overgrow the endoderm at the posterior border of the blastopore, 

 while the mesenchyme cells are overgrown by the muscle cells, 

 and finally the latter by the ectoderm. (Figs. 7-10.) In the closure 

 of the blastopore the anterior (dorsal) lip grows posteriorly until 

 it covers most of the dorsal face, while the muscle cells form the 

 lateral boundaries of the blastopore. (Figs. 9, 10.) In this over- 

 growth of the dorsal lip the chorda cells which originally lay at the 

 anterior border of the egg are carried back into the posterior half 

 of the embryo, where by interdigitation they form the chorda. 

 The neural plate cells are also carried back with the chorda 

 nearly to the posterior end of the embryo. The ventral (posterior) 

 lip of the blastopore then grows forward over the remnant of the 

 blastopore and the neural plate is rolled up into a tube which 

 closes from behind forward. The muscle cells become arranged 

 in three rows on each side of the chorda; in front of the muscle 

 cells is a mass of small mesenchyme cells, while a double row of 

 endoderm cells ventral to the chorda constitutes the cord of ventral 

 or caudal endoderm. (Figs. 11 and 12.) Finally the tail of the 

 larva elongates greatly and becomes coiled around the body of the 



