178 bulletin: museum of comparative zoology. 



There is, however, no other evidence than that of size and position 

 to show which is the sperm and which the egg nucleus. I have found 

 nothing to indicate what the possible outcome of such a condition 

 would be. 



The germ nuclei, whether composed of one or of several vesicles, 

 approach each other, though even at the time of apposition each may 

 be composed of several vesicles. The nuclei shown in Figure 23 as 

 over-lapping are not in contact, a thin layer of cytoplasm occurring 

 between them. The chromatin in the nuclei is in beaded strands, 

 not connected into a network. The nuclei come to lie one against the 

 other, and at the time of apposition may be of equal size (Fig. 24) 

 or unequal (Fig. 25a). The chromatin begins its concentration, 

 preparatory to forming chromosomes, before the membranes of the 

 nuclei disappear (Fig. 25b), and this concentration is first evident on 

 one side of the nucleus. No asters are found in connection with the 

 nuclei at this stage. Whether the definitive chromosomes are formed 

 before the membrane dissolves is uncertain. Figures 24, 25a and 25b 

 show a concentration of the chromatin already in progress, but Figures 

 26 and 27, which show the first cleavage spindle forming, still show no 

 definite chromosomes, even though in Figure 27 there is only a single 

 vesicle, the sperm and egg nuclei apparently having completely fused. 

 In none of these cases are definite chromosomes present. On the 

 other hand, Figure 23 shows distinct beaded strands in both nuclei, 

 as though the chromosomes were already distinct, though not yet 

 condensed into their definitive form. That there may be a complete 

 fusion of the germ nuclei without any^astral figure, seems probable 

 from the conditions seen in Figures 24, 25a and 27, though Figure 26 

 seems to indicate the presence of two more or less distinct groups of 

 chromatin threads and a spindle already nearly formed. 



The cleavage spindles have definite asters, but how these arise I 

 have not been able to determine. We have seen that either the sperm 

 nucleus (Fig. 16a) or the egg nucleus (Fig. 21b) may possess an aster 

 before apposition occurs, but Figures 24 and 25a show the entire 

 absence of asters, as is sometimes the case before apposition. Figures 

 26 and 27 show two asters in each case, and the beginning of the 

 formation of the spindle, but no hint is given of the origin of the 

 asters. 



The conditions described seem to warrant the following conclusions: 

 Spermatozoa usually enter the egg after both polar cells are formed. 

 The penetration may take place at any point on the surface of the egg, 

 but more commonly it occurs in close proximity to the egg nucleus. 



