\KCIM u K \ -FORMATION OK THE GERM-LAYEBS. 



34a 



medullary plate.* This rudiment, when it first appears, consists, at 

 the sides of the still open blastopore, of a single row of cells, while, 

 in front of the blastopore, it is composed of several rows of cells. 

 In later stages (Fig. 154), as the blastopore narrows, the part of 

 the medullary plate in front of it extends more and more, and, at the 

 time when the blastopore is represented by merely a small aperture 

 (Fig. 155), is a large and slightly depressed area. The medullary 

 groove thus formed, which is open anteriorly, is bounded by two 



6 en' 



Fn;. 155. — Later ontogenetic stage of Clavelina Rissoana with the blastopore much 

 narrowed, and the medullary groove appearing (after van Benedex and Julin). A, 

 dorsal aspect; /,', median sagittal section, h, blastopore: <•/', rudiment of tin- 

 chorda; ec, ectoderm; era, entoderm; m, medullary groove ; n, cells of the nerve- 

 ring. 



lateral swellings (medullary swellings, m) which pass into one another 

 behind the blastopore, thus forming a semicircle. 



A differentiation similar to the above is evident in the entoderm 

 (Fig. 158). The cells of the latter, as a rule, are large and turgid, 

 but in the region of the dorsal wall smaller cells appear which are 

 originally arranged so as to form a ring encircling the blastopore, one 



* [According to Castle (No. II.), the cells lying behind the blastopore and 

 marked n in Figs. 154 and 155 are not part of the rudiment of the central 

 nervous system as stated by Jdlin and van Beneden, but are in reality 

 muscle-cells. The rudiment of the nervous system is situated entirely in 

 front of the blastopore. In Ciona the posterior margin of the blastopore does 

 not grow forward over the blastopore, covering in the medullary canal, as 

 described bv van Beneof.n and Julin in the ease of Clavelina. — Ed.] 



