castle: embkyology of ciona intestinalis. 247 



16-cell stage, and was foreshadowed still earlier by the internal constitu- 

 tion of the unsegmented ovum. The endoderni fundament is in the sixth 

 generation^ (Plate XI. Fig. 71, d^-\ d^-\ d^-^, a^-^, a^-^, and the corre- 

 sponding cells in the left half of the Figure); the chorda-mesenchyme 

 ring is chiefly in the seventh generation, though a single pair of its cells 

 has recently passed into the eighth (Z>■•^ B' \ d''-^^ d^-^\ A^-'^, ^«-", 

 — the deep-lying sister cell of ^^"'^j not shown in the Figure, — a'-^\ a'-^^, 

 a'-^°, a'-^, and the corresponding cells in quadrants £ and C) ; the cells 

 of the neuro-muscular ring are all in the eighth generation, except a 

 single pair which lingers in the seventh (D'-^, B^-^*, i)*-', Z>*-^, ^^■^", 

 ^8.15^ j^8.8^ ^8.7^ g^j-,(j |.^Q corresponding cells in quadrants B and C) ; the 

 ectoderm cells are all in the ninth generation, those nearest the animal 

 pole having been the first to divide and pass into that generation. We 

 notice in this stage, as in the earlier stages, a region of delayed division 

 in the equatorial band at the posterior end of the embryo. 



0. Histological. 



Figures 63-70 (Plate X.) represent eight cross sections from a series 

 through an embryo (not figured) a little more advanced in development 

 than the one shown in Plate X. Figs. 61 and 62. The approximate 

 position in the embryo of the sections figured is indicated by the hori- 

 zontal lines on Figui'e 62. 



The differing stainability of cells at this stage, together with other 

 histological peculiarities, serves already to distinguish the fundaments 

 of the various organs with considerable precision. 



The endoderm cells (d^-\ Fig. 64; d^-^, Figs. 65 and 66; «"•«, c?«•^ 

 Fig. 67 ; a*^''. Fig. 68 ; together with the corresponding cells in the left 

 halves of these Figures) are, on account of their slow division, still very 

 large. They are columnar in form, and contain large nuclei. Their 

 cytoplasmic portion scai'cely stains at all except in the region of the 

 nuclei, being almost entirely taken up with close-packed yolk granules. 

 A small amount of protoplasm staining a bright blue in haematoxylin 

 extends out from either side of the nucleus in the long axis of the cell. 

 In this small protoplasmic mass evidently lies an attraction sphere close 

 up to the wall of the nucleus. The nuclei themselves contain numerous 

 chromatic granules. 



The mesenchyme cells (Z)^«, Fig. 64 ; d^-'^^, Figs. 65 and 66 ; ^^^ 

 Fig. 67 ; together with the corresponding cells in the left halves of these 

 Figures) are sharply distinguished from those of every other tissue by 



1 See the table on page 275. 



