CASTLE: EMBEYOLOGY OF OIONA LNTESTINALIS. 261 



To summarize our observations on the series of sections just examined 

 (Figs. 93-97): — 



(1) The fundament of the nervous system consists of a medullary plate 

 extending from near the anterior end of the embryo to the blastopore, 

 aud continued backward by cells lying on each Bide of the blastopore 

 and along the line where the lips of the blastopore have fused. The 

 transformation of the medullary plate into a medullary groove proceeds 

 from the blastopore forward. 



(2) The chorda fundament consists of a plate of cells immediately 

 underneath the medullary plate, but extending neither so far forward 

 nor so far backward in the embryo. A part of it lies on each side of the 

 blastopore, but the larger part is anterior to the blastopore. 



(3) The mesenchyme extends in two lateral bands from the region of 

 the blastopore forward through about two thirds of the extent of the 

 embryo anterior to the blastopore. 



(4) The muscle cells lie principally posterior to the blastopore in a 

 pretty compact mass. They extend no farther forward than the first 

 section anterior to the blastopore. 



(5) The endoderm consists of a double row of large cells ventrally 

 situated extending from the first section behind the blastopore through 

 the next five anterior sections : it then broadens out and occupies nearly 

 the whole inner layer of the embryo, both dorsally and ventrally, anterior 

 to the chorda fundament. 



C. Summary on Gastrulation. 



1. In the gastrulation of Ciona two processes can be distinguished : 

 (a) a progressive invagination of the cells on the dorsal surface of the 

 embryo, beginning at its centre; (b) a concomitant overgrowth of cells 

 from the ventral side of the embryo, caused by more rapid cell division 

 in that region. The overgrowth is greater at the anterior than at the 

 posterior end of the embryo, because cell division proceeds more rapidly 

 at the anterior end. 



2. Early in the process of gastrulation one can recognize a ring of 

 cells encircling the blastopore peculiar in their stainability, forming the 

 common fundament of the nervous system and the longitudinal muscu- 

 lature of the larva. 1 Anterior to the blastopore the ring broadens out 



1 The existence of this peculiar ring of cells was first pointed out by Van Rene- 

 den et Julin ('86) in the case of Clavelina ; but these authors made the mistake of 

 regarding it as exclusively nervous. 



