132 



OHAS. B. WILSON. 



already been given in reference to the early development of 

 the ovum, furnish strong corroborative evidence to the truth 

 of the view formulated by Mark in 1881 (32) and by Watase 

 ten years later (47). The ovum is at first isotropic, but after- 

 ward acquires polarity and other promorphological features 

 through its topographical relation to the remaining cells of 

 the maternal tissue. In this species the primary axis of the 

 ovum seems to be formed at right angles to the surface of 

 the epithelium from which it originates (cf. 50). 



The appearance of the first polar body is accompanied by 

 the usual flattening of the surface, and sometimes by a slight 

 pitting in of the ectosarc to meet it (fig. 23a). The latter 

 effect is often abnormally increased if the cover-glass is 

 allowed to press upon the egg. The gradual emergence and 

 rounding of the first polar body is shown in figs. 24 — 26. 



A slight swelling first appears at the point of emergence, 

 and rises as a symmetrical hemisphere. This increases in 

 size, and within a minute becomes a short cylinder, or a more 

 or less rounded conical protuberance (fig. 24). 



Within half a minute more the base of the cylinder is con- 

 stricted, and the constriction increases until the polar body 

 has entirely separated from the egg (fig. 26). So far as 

 observed a connecting protoplasmic band is always present, 

 but it requires a good light and a high power to detect it. 



The flattening of the pole draws the surface of the egg 

 away from the membranes, so that the polar body after its 

 extrusion is not very near them. It now becomes approxi- 

 mately spherical, the whole formation having occupied about 

 three minutes. 



It lies in contact with the surface of the egg or near it for 

 ten or fifteen minutes, during which the egg resumes its 

 original shape, and then withdraws a little, but still retains 

 the connecting band of protoplasm. This withdrawal is 

 simultaneous with the first appearance of visible spin-threads, 

 and marks the beginning of a long series of remarkable 

 activities which are of sufficient importance to demand a 

 separate description (see p. 133), 



