44 PROCEEDINGS OF THE AMERICAN ACADEMY 



that the shadowy ring is due to the polar aggregation of the proto- 

 plasm. The clear central space is the place in which the pronuclei 

 will soon become visible. Our observations on the formation of the 

 polar globules are not yet completed ; and for the present may be 

 passed over with the single remark, that these bodies do not escape 

 through the micropyle in the case of Ctenolabrus. 



At the time the pronuclei appear, the blastodisc has a low conical 

 form, the rounded summit of which is directed towards the centre of 

 the vitellus. The peripheral, basal portion scarcely swells beyond 

 the niveau of the egg-sphere, and its margin thins out gradually into 

 the periblast. There is a thin, but distinct perivitelline space separat- 

 ing the zona radiata from the blastodisc and the upper half of the 

 periblast. The axis of the blastodisc is vertical, coinciding with that 

 of the ovum. One or two polar globules are seen in the perivitelline 

 space, at the octodermic pole. The fluid fdling this space is not pure 

 water, as shown by the fsict that it becomes finely granular in acids, 

 and stains in carmine solutions. 



Pronuclei. — The earliest view of the pronuclei that we have ob- 

 tained represents them as two equal spheres (.01 mm. in diameter) 

 already in contact. The male pronucleus lies directly above the female, 

 so that a line joining their centres would pass through the point of 

 contact and coincide with the axis of the ovum. These bodies have 

 a smooth and distinct outline, and a slightly granular composition. 

 In general appearance they do not differ from the nuclei of the 

 blastomeres. 



In a mounted preparation of the blastodisc, which was killed thirty 

 minutes after the ova and spermatozoa were mixed, the male pronu- 

 cleus lies at some distance above the female pronucleus. The latter 

 is spherical (.007 mm.) and surrounded by radial lines ; while the 

 former has an elongated ovate form (.009 X -004 mm.), the larger end 

 of which is nearest the female pronucleus. This form of the male 

 pronucleus recalls that of the " male aster," as described by Fol ■*' in 

 the ovum of Sagitta Gegenbauri. 



AVe have several times watched the conjugation of the pronuclei 

 continuously, from the moment of contact to that of complete coales- 

 cence. The two spheres flatten against each other, the line of junc- 

 tion remaining distinct until each has assumed the form of a hemi- 

 sphere. This stage is reached in seven minutes after first contact. 

 The line of junction now becomes obscure, first in the middle, then at 



*^ Fol. liecherches sur la Fc'condation, etc., 1879, PI. X. figs. G and 7. 



