EFFECT OF HEAT ON THE EGG OF CUMINGIA. 263 



to the egg nucleus. The aster has already divided and the 

 spindle is forming before the male and female pronuclei have 

 fused. In some cases, apparently, the pronuclei do not unite at 

 all; and one finds such a prophase of the first cleavage as that 

 represented in Fig. 5. Here two skeins of chromatin are present 

 in the spindle, and the figure shows what it is important to note, 

 namely, that even if the pronuclei do not unite before cleavage, 

 all chromatic material is included in the formation of the chromo- 

 somes of the first cleavage spindle. 



We can trace, then, in eggs which have been heated imme- 

 diately after fertilization, a suppression of maturation which 

 leads to the formation of two daughter nuclei within the egg. 

 Further, we can trace the union of both these nuclei with the 

 male pronucleus, leading to the formation of a cleavage spindle 

 which should contain eighteen chromosomes from the sperm 

 and thirty-six from the egg. This suppression of maturation is 

 exactly like the process which has been seen to take place in the 

 unfertilized eggs, in which, owing to the absence of the sperm, 

 the result is a diploid instead of a triploid amount of chromatin. 



(c) The First Cleavage of Heated Eggs. The first cleavage 

 spindle forms normally in the eggs which have been heated 

 immediately after insemination. Before its formation the eggs 

 have been returned to cooler sea water, so that conditions sur- 

 rounding them are normal during the division. The important 

 variation from the usual condition is seen in the chromatic part 

 of the figure. Theoretically, of course, there should be fifty-four 

 chromosomes, but as a matter of fact there are sometimes fewer, 

 sometimes more than this number. Figs. 6 to 9 show plates of 

 chromosomes from such eggs, in which the numbers are 44, 48, 

 50 and 61, respectively. 



It is in a comparison of these plates with those from partheno- 

 genetic, normal and dispermic eggs that the greatest interest is 

 to be found. The equatorial plates of the first cleavage of normal 

 eggs show how much the form of the chromosomes is modified 

 by the experimental treatment. In the normal eggs (Figs. 10 

 and n), one finds long threads so much twisted about each other 

 that accurate counting of them is impossible. This is the case 

 also in the dispermic egg, as shown in Fig. 12. It is evident that 



