EFFECT OF HEAT ON THE EGG OF CUMINGIA. 267 



together in the spindle. Fig. 18 is from a fertilized egg in which 

 maturation has been suppressed, Fig. 19 from a normal one, 

 and Fig. 20 from one which was heated after its polar bodies 

 had been formed. A comparison of the three figures shows that 

 there are approximately the same number of chromosomes in 

 the plates represented in 19 and 20, while 18 shows a larger 

 number. This is in accordance with the belief that the polar 

 nucleus contributes normal chromosomes to the egg. 



Further evidence is given by a study of the resting nuclei. 

 Boveri ('05) has shown that the surface area of a resting nucleus 

 is proportional to the number of chromosomes in it, and this 

 fact is frequently used in determining the relative amounts of 

 chromatin when the cleavage chromosomes cannot be counted. 

 In the present 'instance, the four cell stage of the various types 

 of egg has been selected for comparison. Here we can be sure 

 of comparing equivalent nuclei, as the size and position of the 

 cells makes them easily recognizable. It will be remembered 

 that the cleavage in Ciimingia is not equal. The first division 

 separates a small cell AB from a larger one, CD. Then follows 

 a division of AB giving rise to a three-cell stage; and next CD 

 divides. In the four-cell stage A, B and C are approximately 

 equal, while D is larger than any of them. In the normal egg, 

 shown in text-figure 2, A, the nuclei of all four cells are equal. 

 Fig. 2, B, shows a parthenogenetic egg, with nuclei like those 

 of the normal one ; and such nuclei are also found in eggs heated 

 after maturation, as shown in Fig. 2, C. All these eggs contain 

 the diploid amount of chromatin, which is not actually changed 

 by the heating, and, as far as can be judged from the resting 

 nuclei, the chromatin supplied by the polar nucleus is equivalent 

 to that supplied by the sperm. Similarity is also found among 

 triploid eggs, whether these contain an extra sperm or a sup- 

 pressed polar nucleus. Text-figure 2, D, shows a dispermic egg 

 with two multinucleate cells and one nucleus abnormally large. 

 Text-figure 2, E, represents a fertilized egg in which the polar 

 nucleus was retained ; here also are two multinucleate cells. In 

 F which is from an egg of the same sort, the cells are not 

 multinucleate, but two of the nuclei are above the normal size. 



(e) The Size and Number of the Chromosomes. From the 



