EMBRYOLOGY OF CLEPSINE. 221 



the germ, vesicle has trebled, and that of the germ, dot doubled. 

 As the egg increases in bulk, becoming more and more opaque 

 from the accumulation of granules, it begins to project from the 

 egg-string, and soon, driving the membrane of the string before 

 it, comes to occupy a position quite external to the neighbouring 

 cells (fig. 59). At this stage the egg measures about '-lO mm., 

 and the vesicle, which can no longer be seen, but the presence of 

 which is indicated by alight spot (by transmitted light), measures 

 •05 mm. At this time 1 have generally found two germ, spots. 



In one case three were found, measuring respectively "005 

 mm., 'OOS? mm., and '0025 mm. The membrane of the egg- 

 stri))g, which is pushed before the Q%^ in its outward growth, 

 forms a constriction (fig. 59), whicli becomes progressively 

 smaller, till the egg is merely pendent from the string. By the 

 time the t^^ has attained a diam. of "55 mm. to '60 mm. (germ. 

 ves. = '06 mm., germ, dot = '008 mm.) it bursts the membrane 

 and falls into the ovary. This event generally takes place at the 

 end of eight to twelve days from the time the growth of the 

 primary egg-cell began. The time varies much according to the 

 temperature. In three to four days more the egg attains its full 

 size. 



The full-grown egg varies much in size, not only in different 

 species, but also in different individuals of the same species. 



The average for C. marginafa is : 



Ei?? = "SO mm. X "75 mm.; germ. ves. =^ "OS to "09 mm.; germ, dot 

 = 012 mm. 



Eor C. complanata about the same. 



The ^^j,2^ of C. bioculata and C. heteroclita measures about 

 •55 X '50 mm. 



The germinal vesicle of the mature egg lies excentrically, 

 sometimes near the surface, and possesses a distinct membrane. 

 Its transparent fluid contents, after treatment with osmic acid 

 and alcohol, appear to be very finely granular (fig. 60, b), and 

 slightly blackened by the acid. The germinal dot is composed 

 of several highly refractive pieces. 



Formation of the Yolk-spheres. — According to Leydig (i-of), 

 the large yolk-spheres result from the coalescence of the minute 

 yolk-granules. I have never been able to discover any indica- 

 tions of such a consolidation. The fact that the refractive power 

 (if these spheres diminishes as the size increase is the reverse of 

 what we should expect as a result of simple consolidation. 



At^ the time the egg falls from the egg-string the largest 

 spheres measure no more than "0075 mm. In the full-grown 

 egg all sizes are found from 02 mm. down to "0005 mm. — the 

 Size of the yolk-granules. The spheres have become very 

 numerous, forming the larger part of the egg-contents. They 



