Maturation and Fertilization in Theridium. 245 



which can be distinguished neither spheres nor centrosomes. Possibly 

 cytoplasmic strands radiate out from this mass to the peripheral 

 cytoplasm, but they must be exceedingly delicate because I have 

 been unable to trace them between the yolk spherules and the 

 central mass appears to be quite sharply bounded. 



In eggs aged from 3 to 3^/o hours prophases of these pronuclei 

 are found, and the changes in the two pronuclei occur simultaneously. 

 Fig. 32 illustrates an early prophase where one pronucleus partially 

 covers the other. Fig. 33 represents the early cleavage spindle with 

 the paternal and maternal chromosomes in separate groups. There 

 are central spindle fibres as well as mantle and polar fibres, but no 

 distinct centrosomes. The upper half of this figure shows some 7 

 or 8 chi-omatin loops and the lower half 11. Evidently the chromo- 

 somes have not yet attained their complete segmentation and conden- 

 sation for their contours are uneven and their grouping is still like 

 that of a spirem, even though the nuclear membrane has entirely 

 disappeared and the spindle is well formed. Similar unusual retar- 

 dation in the condensation of the chromosomes is also to be found 

 in the mitoses of the cleavage cells, as shown in two spindles of 

 the 4cell stage, Figs. 35, 36. Fig. 34 shows the two chromosome 

 groups in another egg of the same stage from the same cocoon. The 

 perfected equatorial plate stage of this first cleavage I was not 

 successful in finding, though a large number of eggs w^ere sectioned 

 for this end.. But that the definitive number of chromosomes in the 

 first cleavage is 24, i. e., that the sperm nucleus probably introduces 

 12 chromosomes the number certainly possessed by the egg nucleus, 

 is shown by Fig. 38 where 24 chromosomes are to be counted in a 

 daughter plate of an anaphase of the 4cell stage. Fig. 37, an equa- 

 torial plate of the 4cell stage, exhibits the chromosomes in their 

 shortened form. 



Distinct cell boundaries between the cleavage cells do not 

 become apparent until the late cleavage, and it has been mentioned 

 earlier that before cleavage the superficial cytoplasm is not cut up 

 into cell areas. 



A main reason why this particular study was undertaken was 

 the hope of finding in the cleavage some morphological signs of 

 somatic differentiation such as have been discovered for Ascaris by 

 BovERi and others, for Dytiscus by Giardina, and for Copepods by 

 Hacker; that is, the hope of tracing the point of divergence of 

 body and germ cells. But the only evidence bearing on this matter 



