266 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



cut off, all of which are ectomeres. Then a fourth set of 

 four (4a, 45, 4c?, 4d) is cut off from the lower cells. 

 One of these four cells (4<f) is very large ; it is 

 the mesoderm cell, or somatoblast, and it soon sinks below 

 the surface into the cleavage or segmentation cavity. The 

 other three cells of the fourth quartette (4a, 45, 4c), and 

 the four lower cells (4:A, \B, 40, 4D) are the endoderm 

 cells, and they occupy a position at the lower or posterior 

 end of the larva. At this time the larva consists of (1) 

 about 50 ectomeres, products of division of the cells of the 

 first, second and third quartettes ; (2) the seven endoderm 

 cells mentioned above, and (3) a mesoderm cell. At the 

 end of twenty-four hours* after fertilisation, the larva, 

 when seen from the lower pole, has the appearance shown 

 in fig. 73. The ectoderm cells are bjr this time more 

 numerous (about TO to 80), and the endoderm cells, dis- 

 tinguished by the numerous yolk granules in their proto- 

 plasm, are in this case thirteen in number, each of the 

 seven primary endoderm cells, except 4c, having divided 

 into two. Lying in the segmentation cavity, and not 

 shown in the figure, are two mesoblast cells produced by 

 division of the primary mesoblast cell i\d). The larva, 

 now a blastula, is converted into a gastrula by groAvth of 

 the ectoderm cells over the 3^olk-laden endoderm cells. 

 About twenty-eight hours after fertilisation the anterior 

 ciliated band is just recognisable and the larva? were found 

 to be rotating slowly within the vitelline membrane. The 

 stomodseal invagination had also made its appearance 

 (fig. 74). Twenty-four hours later both the ciliated bands 

 of the larva were well marked, a broad one just anterior to 

 the mouth, and a narrow one near the posterior end of the 

 animal, and by this time the rotatory motion was much 



* It is very probable that in shallow water, in sunlight, the 

 development would be more rapid than it was in the cool laboratory. 



