70 FECUNDATION ; NON-MOTILE ISOGAMETES. 



furnished by the male gamete. Chmielewskij ('90) finds that in all of 

 the several species examined the chloroplast of the male gamete is 

 dissolved in the zygote, that of the female only remaining. 



The behavior of the nuclei during fusion cannot be followed with 

 any degree of certainty in the living specimen. As a rule they cannot 

 be seen at all, a fact which led to the view of the earlier observers that 

 the product of union was without a nucleus. One must, therefore, 

 resort to thin and well-stained sections of properly fixed material to 

 observe the details of nuclear fusion. For this purpose I have selected 

 a small-celled species with one chlorophyll band. 



When the young zygote is provided with a thin cell-wall, the two 

 nuclei, which are exactly alike, judging from their appearance, are 

 seen lying closely applied to each other (Fig. 20, B). Each contains 

 a rather large and distinct nucleolus and the characteristic limn net in 

 which are imbedded small granules that behave toward stains as 

 chromatin granules in resting nuclei of higher plants. In fact, the 

 nuclei of Spirogyra in this condition seem to possess the same 

 structure as the phanerogamic nucleus. The contiguous parts of the 

 nuclear membranes dissolve or disappear as such, and the network of 

 the one unites directly with that of the other, the fusion of the nucleoli 

 following later (Fig. 20, C). Frequently, before complete union of the 

 nuclei, the wall of the zygospore may become much thickened and less 

 easily penetrated by fixing fluids, so that perfect preparations are difficult 

 to procure. During the development of the zygospore the chloroplasts 

 become vacuolate and the identity of each cannot be made out. 



In the preceding paragraphs I have described the nuclear fusion in 

 the zygote as I was able to follow it, but for lack of time and suitable 

 material an exhaustive study of the subject was not made, and conse- 

 quently I am not prepared to state whether the peculiar behavior of 

 the nuclei as described by Chmielewskij ('92) for Spirogyra crassa 

 and S. elongata is correct. Chmielewskij states that, as the gametes 

 round up, the nuclear membranes become less distinct, disappearing 

 entirely as the gametes unite. The nuclei now fuse, the fusion being 

 complete by the time the zygote is provided with a thick, dark wall. 

 This fusion takes place during the prophase of division. As soon as 

 fusion is complete the nucleus divides. The daughter nuclei now 

 divide, four nuclei resulting. Two of these then fuse, while the other 

 two divide by direct division and finally disorganize. The fusing 

 nuclei are provided with membranes and are in the resting condition. 

 If the observations of Chmielewskij be true, the process in Spirogyra 

 is without parallel in the plant kingdom, at least so far as the author 

 is aware. 



