506 A. M. (JAlili SAlJNDEliS AND MARGARET POOLE. 



41); gastrulation is of the epibolic type, and the blastopore 

 is formed at the vegetative pole. It narrows to a slit-like 

 opening, diminishing in si/e by the continual growth of the 

 anterior and lateral parts of the ectodermal sheath, but does 

 not close completely, but persists as the mouth. At the end 

 of segmentation, owiug to the large size of the endomeres 

 A and B the embryo becomes somewhat heart-shaped. Between 

 the large endomeres a small space appears, the segmentation 

 cavity, which is more or less triangular in shape in optical 

 section, the broadest end being towards the posterior end of 

 the embryo. Two ectoderm cells, 2d~'"~^ and 2d-~~~'~, increase 

 greatly in size and come to project from the surface. These 

 are known as the anal cells. Uurinof sesrmentation there 

 is a shifting of the embryonic axis, and these cells come, 

 in consequence, to mark the posterior end of the larvae. 

 At the opposite end the velum is formed as a simple ring iq 

 the region of the B quartette. By the time the cilia are deve- 

 loped the embryos begin to rotate within their capsules. At 

 this stage there are about two hundred and fifty cells. 



Text-fig. 7 represents an embryo with about one hundred 

 and seventy cells, seen in optical section, from the vegetative 

 pole. The blastopore is now reduced to a narrow slit, and 

 posteriorly the anal cells project from the surface. Anteriorly 

 the polar bodies were present still adhering to the embryo, 

 but are not represented. Internally the two large endomeres 

 divei-ge from one another to leave the segmentation cavity 

 between them, while the derivatives of the much smaller 

 endomei-es C and D are shown. The mesomeres, which are at 

 this stage eight in number, stretch across from the anal cells 

 towards the position occupied by the blastopore, which is not 

 represented. 



anteriorly to give rise to the mesodermal bands. After the formation 

 ■of the three quartettes of micromeres a fourth generation is produced 

 hy A, B, C ; this consists of 4 A, 45, 4C. which go towards the forma- 

 tion of the endoderm. The above lineage is given in tabular form by 

 Robert (26), to which the reader is referred for the detailed analysis of 

 the later segmentation stages. 



