ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 931 



exactly as it does after fertilization, and such ova cannot be fertilized by 

 spermatozoa. 



In investigating tbis matter a little further it is necessary to distinguish 

 the two peculiarities of protoplasm which are exhibited in the formation 

 of the yolk-membrane ; these are its secretory activity and its irritability. 

 The egg must have a minimum stimulus to produce its membrane, and this 

 minimum stimulus is, in normal eggs, the entrance of one spermatozoon. 



We now pass to the changes in the conjugation of the sexual nuclei 

 and the internal processes of fertilization. The experiments which have 

 been made show that, with the aid of reagents, the copulation of the nuclei 

 may be hindered or stopped. It seems to be certain that nuclei provided 

 with all the vital properties which are necessary for further development 

 only appear when the substances are thoroughly impregnated by the male 

 and female nuclei ; however, even when the nuclei do not unite, they have 

 properties which they had not originally ; the male and female nucleus are 

 both capable of undergoing fibrous differentiation and forming chromatic 

 loops and a chromatic filament, even if they are kept separate from one 

 another. If portions of ova without nuclei are separated from the rest of 

 their cell, they may be penetrated by spermatozoa which in them form 

 spindles ; or, in more general terms, we may say that the protoplasm of the 

 egg alone is able to give the male nucleus the power of forming spindles. 



In ova, however, which possess germinal vesicles, the spermatozoa 

 undergo no changes and are not acted upon by the protoplasm of the egg ; 

 if the directive spindle is in the process of formation the heads of the 

 spermatozoa remain unaltered, but there is a slight radiation of the proto- 

 plasm. No exchange of substance takes place between the male nucleus 

 and the ovarian protoplasm until after the formation of the first directive 

 corpuscle. 



In observing the fate of the female nucleus the authors foimd three 

 classes of results : the ovarian nucleus copulates with only one male nucleus, 

 and in division only one spindle is formed ; the ovarian nucleus copulates 

 with two or more male nuclei, and produces four-poled or many-poled 

 karyokinetic figures ; or the ovarian nucleus remains by itself, and by the 

 imbibition of fluid increases rapidly in size. The last phenomenon is 

 common in proportion to the number of spermatozoa that enter the egg. 



Two or three male nuclei may certainly fuse with the female nucleus, 

 BO that the capacity of the female nucleus for receiving spermatic nuclei 

 appears to be considerable, and to last even after several copulations have 

 taken place. The male nuclei undergo fibrous differentiation, and become 

 converted into small spindles, which in course of time also divide; 

 nothing, however, is yet known as to the fate of these products of division. 



The last point to be noticed concerns the variations in the phenomena of 

 cleavage. In their experiments, the Drs. Hertwig altered the process of 

 cleavage in three ways ; the eggs, after fertilization, were treated with 

 reagents, or they were fertilized by several spermatozoa, or the completion 

 of fertilization was hindered. In the normal stages of cleavage two 

 processes go on simultaneously in the nucleus ; one is an increase in size, 

 and the other is karyokinesis. The latter, but not the former, is affected 

 by quinine and chloral ; similar changes may be seen in polyspermy. When 

 this last is brought about by the aid of morphia, strychnine, or nicotine, 

 or, in other words, by reagents which do not of themselves influence the 

 process of division, there appear tetraster or poly aster figures, which are 

 to be explained not by the action of the reagents, but by polyspermy. There 

 can be no doubt that fertilization by two spermatozoa leads to the forma- 

 tion of tetrasters ; but all tetrasters must not be referred to this cause, as 



