ORGAN-FORMING SUBSTANCES IN EGGS OF ASCIDIANS. 215 



this overgrowth of the ventral lip the transverse rows of muscle 

 cells again assume an antero-posterior direction in the embryo 

 (Photos 1 8-21). 



Since the neural plate is composed of relatively transparent 

 cells which overlie the gray chorda plate, it is not well shown in 

 the photographs, unless seen in profile (Photos 22, 23). In the 

 dorsal view shown in Photo 18 seven or eight transverse rows of 

 cells may be indistinctly seen in the neural plate. The chorda 

 plate, which contains considerable yolk and is gray in color lies 

 under the neural plate. It consists at first of a single transverse 

 row of eight cells, then by the division of these cells two such 

 rows are formed and finally by shoving together from the sides 

 this plate becomes much narrower and longer. In later stages 

 the chorda plate and the neural plate push back between the 

 muscle cells of each side until they reach the hinder end of the 

 embryo, thus establishing the characteristic appearance of the 

 young larva shown in Photo 21. It can now be seen distinctly 

 that the deep yellow cells have become the lateral muscles in 

 the tail, that the light gray cells of the chorda plate have formed 

 the fusiform chorda which lies between the muscle cells, and 

 that the deep gray cells form the gastral endoderm. At the 

 posterior end of the chorda is a group of light yellow cells which 

 connect the muscle rows of the right and left sides ; these are 

 the caudal mesenchyme cells. At the anterior ends of the 

 muscle rows are the clear areas of the trunk mesenchyme, which 

 are also of a faint yellow color, while around the entire embryo 

 is a clear layer of ectoderm cells (Photo 21). 



The form of the larva is now well established and subsequent 

 development changes this form only in minor features. In Photo 

 23 the tail is much elongated and bent toward the ventral side. 

 Three rows of muscle cells with clear nuclei can be seen on the 

 left side of the tail, while the larva is tilted toward the left so 

 that the dorsal row of muscle cells of the right side is also visi- 

 ble ; with the elongation of the tail the individual muscle cells 

 have become much longer than in previous stages. Between 

 the dorsal rows of muscle cells of the right and left sides is a 

 clear line which is the neural tube ; anteriorly this tube overlies 

 the dark gray endoderm and hence it is not clearly visible in the 



