The Development of Phascolosoraa. 93 



partially express the facts, for in the posterior hemisphere trom the 

 eight-cell stage onward, the cells of quadrant D occupy practically 

 the whole of the dorsal side. The manner of cleavage in Fh. gouldii 

 and in Ph. vulgare is almost identical, as far at least as the 16-cell 

 stage. The descriptions of the later stages of segmentation are 

 based upon the study of Ph. vulgare. 



The most striking features of the cleavage are (1) the small 

 size of the "macromeres" in the eight-cell stage, the major part of 

 the yolk being concentrated in the first group of "micromeres", and 

 through them transmitted to the trochoblasts. (2) The alternating 

 directions of the cleavage planes up to forty-eight cells, and in 

 certain regions of the Qgg still further, are in accord with the 

 ordinary type of spiral cleavage. In the division of the intermediate 

 cells in the completion of this stage, however, the direction is 

 radial, but shows a spiral tendency. (3) The rosette, cross, and inter- 

 mediate cells, which are characteristic of Molluscs and Annelids, are 

 present in the 48-cell stage. (4) The mesoderm pole cell is given 

 off by 4d, the fourth micromere that arises from the large cell, B, 

 of the four-cell stage. 



First Cleavage and Two- cell Stage. 



The spindle of the first cleavage lies horizontally in the midst 

 of the region of finely alveolar protoplasm, which extends from the 

 active pole through the centre of the ^^^. It lies, therefore, slightly 

 nearer the active than the passive pole. 



The first cleavage furrow makes its appearance about one hour 

 and forty minutes after first contact with the spermatozoa, in eggs 

 of Ph. vulgare which are laid under ordinary conditions. Three hours 

 may elapse before eggs taken from the body cavity and artificially 

 fertilized reach this stage. The difference in time is due to the 

 fact that eggs in the coelom, unlike those thrown out from the 

 nephridia, are only occasionally found with the spindle of the first 

 polar body already formed. A flattening of the cytoplasm at the 

 active pole first appears; then a slight depression beneath the polar 

 bodies deepens into a furrow, which extends rapidly downward toward 

 the vegetative pole. This divides the a^^ into blastomeres the 

 volumes of which are about as one to three (Fig. 20). This furrow 

 does not pass through the actual vegetative pole, which lies on the 

 lower side of the larger cell. 



