castle: embryology of ciona intestinalis. 257 



blastopore (cf. Plate XI. Fig. 80). On its depressed dorsal surface are 

 seen six cells of the anterior chorda fundament, which is being rapidly 

 covered over from the sides and anterior end by the ectoderm. Extend- 

 ing deep down on either side of the chorda appears a deeply stained cell 

 (stippled in the drawing) with large nucleus. These two are the most 

 posterior cells of the medullary plate, which now lies at the dorsal surface 

 of the embryo anterior to the blastopore, having been formed chiefly by 

 the anterior segment of the neuro-muscular ring (cf. Plate XL Fig. 80). 

 The mesenchyme cells, A^-^^, B^--* (Fig. 87), are seen to lie on each 

 side of the gastrula cavity (cf. Plate XI. Fig. 82) ; lateral to them lie the 

 relatively small and faintly stained mesenchyme cells, A^'-'^, B^-^^. The 

 respective sister cells of those just mentioned, viz. A^''^, B^-'^^, A^-'^^, and 

 B^-^^ (cf. Plate XI. Fig. 82), lie in the next two posterior sections (not 

 figured). 



The stage next to be discussed differs in external appearance from that 

 shown in Figure 80 chiefly, first, in the further contraction of its blasto- 

 pore to a small aperture in the dorsal surface somewhat posterior to its 

 centre ; secondly, in a slight elongation of the embryo and narrowing of 

 its posterior end, foreshadowing the formation of the tail ; and thirdly, 

 in a slight depression of the medullary plate to form a neural or medul- 

 lary groove (cf. Fig. 98). 



From a series of transverse sections through an embryo in this stage, 

 five are represented in Plate. XII. Figs. 88-92. Figure 88 (Plate XII.) 

 shows a section posterior to the blastopore (cf. Fig. 98). It passes 

 through the region of the small posterior mesenchyme cells, C'^, ZP-^. 

 Lateral or dorsal to them are seen four pairs of muscle cells containing 

 large nuclei. The finely granular cytoplasm of these muscle cells takes 

 a deep grayish blue stain in hematoxylin. Bounding the whole section 

 is the uninterrupted ectoderm. 



The next anterior section, which has nearly twice the area of this, 

 is likewise completely surrounded by ectoderm. 



Tlio second section anterior to the one shown in Figure 88 is repre- 

 sented in Figure 89 (Plate XII.). Two endoderm cells, the most pos- 

 terior ones, appear in it. The small size of their nuclei indicates that 

 they belong to a later generation than the endoderm cells seen in Fig- 

 ure 84. Unquestionably they are in the eighth generation. To right 

 and left of them appear two muscle cells, probably descendants of C''^, 

 Z)^■^ Plate XL Fig. 73 (cf. Plate XL Fig. 83, C^", C'-'^ Z)^", and 

 -D^'^^). Lateral to the muscle cells mentioned are seen in Figure 89 



