536 CEPHALOCHORDA. 



membrane becomes greater. There is no micropyle. The spermatozoa 

 pass through this elastic membrane to reach the egg.* 



The first stages of development closely resemble those of the 

 Ascidians. Cleavage is total and almost equal (adequal type of 

 HATSCHEK). The first furrow is meridional and appears first at 

 the animal pole, where for some time it is deepest ; it eventually 

 divides the egg into two exactly equal parts (Fig. 279 B). The 

 second furrow, which is also meridional, is at right angles to the first 

 and leads to the rise of four blastomeres of equal size which leave free 

 between them a cavity open above and below ; this is the cleavage- 

 cavity (Fig. 279 C and Z>). The eight-celled stage (Fig. 279 E) is 

 brought about by an equatorial furrow which lies somewhat nearer 

 the animal than the vegetative pole and leads to the first differentia- 

 tion between the blastomeres of the animal and vegetative halves. 

 The embryo consists of a circle of four smaller blastomeres near the 

 animal pole and another circle of four larger blastomeres belonging 

 to the vegetative pole. Further meridional furrows divide these eight 

 cleavage -spheres into sixteen, the sixteen-celled stage then consisting 

 of a circle of eight smaller and another of eight larger blastomeres 

 (Fig. 279 F). 



Even at this sixteen-celled stage, according to WILSON, certain individual 

 differentiations are found which influence the further course of cleavage. The 

 regular stage described by HATSCHEK, in which the eight cells of the upper 

 circle are found resting regularly on the eight cells of the lower circle, was 

 comparatively rarely observed by WILSON. The blastomeres of the upper 

 circle often appear shifted spirally in relation to those of the lower, as is found 

 to be the case in the Annelida and Mollusca. Most frequently, however, at 

 the sixteen-celled stage a bilateral (or strictly speaking a biradial) symmetry 

 is evident in the arrangement of the blastomeres, the eight cells of the vege- 

 tative half being divided into four larger and four smaller cells. The four 

 larger cells surround the vegetative pole in regular order, while the four 

 smaller cells are grouped in two pairs symmetrically to the median plane. 

 This median plane, according to WILSON, corresponds to the first cleavage- 

 plane. A similar arrangement of the blastomeres was seen in the sixteen-celled 

 stage of the Ascidians. 



The thirty-two-celled stage arises, according to HATSCHEK, in 

 consequence of further equatorial furrows. It consists of four rows 

 of eight cells each, one super-imposed above the other. The dilated 

 cleavage-cavity, which was hitherto open at the animal and vegetative 

 poles, now becomes closed at these points. In the further course of 



[* Only one polar body is generally said to be present at this stage (Fig. 

 279), but SOBOTTA (No. XI.) has recently discovered the presence of a second 

 one. ED.] 



