The life history of Diplodiscus temporatus Stafford. 633 



close of the period of constriction, and the subsequent breaking of 

 the strand of tissue formed by their fusion. From the appearence 

 ot the cell wall as is shown in Fig. 38. it would seem that there 

 Avas an active pushing out of the nucleus at the time of the 

 extrusion of the polar body. In this egg the cell wall extends up 

 around the narrowest part of the constriction between the nucleus 

 and the polar body, just as would be true for any elastic membrane 

 which had been ruptured by pressure from within. 



In only a single instance among all the eggs with polar bodies 

 found among my material, was there any evidence of the division 

 of a polar body after its separation from the egg. In the egg shown 

 in Fig. 41, there are two small bodies attached, both of which 

 have the characteristic appearence of polar bodies. There is also in 

 favor of the conclusion that these bodies came from the division 

 of a single one the fact that taken together they make a mass about 

 the size of an ordinary polar body. 



A comparison of the several figures of eggs with polar bodies, 

 shows that there are considerable differences between the sizes of 

 the latter. If my interpretation of the method of the extrusion of 

 the polar bodies is correct, the difference in their size is not sur- 

 prising. Prof. CoNKLiN, in a paper as yet unpublished, points out 

 the fact that in the eggs of Crepidula the size of the polar bodies 

 can be controlled by differences in pressure. Within the sporocysts, 

 or rediae, of the Malacocotylea while they are within the body of 

 the host, there must be considerable variations in the pressure from 

 time to time, so in the light of the results just quoted, this fact 

 might have some considerable influence in determining the size of the 

 polar body cut off from any egg. 



The maturation phenomena just described conform to the type 

 generally occurring among parthenogenetic eggs from which only one 

 polar body is given off. There is no numerical reduction of the 

 chromosomes. In the maturation division the chromosomes are 

 halved, just as in a segmentation division, in other words the divi- 

 sion is purely quantitative. 



2. The segmentation of the egg and the formation of 



the germ layers. 



The nuclear phenomena concerned in segmentation have been 

 described in detail in an earlier section of this paper and need not 

 be discussed in this connection. 



