EABLY DEVELOPMENT OF GRAFFILLA. I Si) 



been found. In a tr\v <>i these the egg nucleus is in a resting 

 condition, thus indicating that the second division may not 

 follow immediately upon the first. However, I have not yet 

 succeeded in finding the spindle of the second polar body divi- 

 sion, but th.it a second polar body is thrown off is clearly shown 

 in ai It -.1-1 one case (Fig. 25). Here the constriction of the second 

 polar body has just been completed, while the first polar body 

 liaxini: undergone division is in the process of disintegration. 

 The rapid disappearance of the polar bodies immcdiatrK after 

 the\ are vi\-ii off has added to the difficulty of studying their 

 formation, a-, well as to the study of the formation of tin- lir-t 

 micromere. 



iVrli.ip- ihc most striking feature of maturation on druffilhi 

 is i In- large size of the first polar body. This i- not surprising : 

 foi it i- noi uncommon for a large polar body to bi- :J\en off in 



tin i tain Hat worms. It was in the egg of a turbellarian, 



Prostth , that Francotte, '97, discovered the intere-tin^ 



lai i that tin- first polar body may be nearly as lar^c as the < 

 it-ell, ami may occasionally be fertilized and d-\elop into a 

 .small iM-trnla, after having first formed a small polar body like 

 the MI i .n< 1 c tin- .it the egg. 



5. l-i-rtilizutiini und the First Cleavage. Fcrtili/ation follou- 

 alnio-t immrdialfly upon the throwing oil of the -,-,-,, D,| p.ilar 

 bil\ . I ha\e found no exceptions to the rule that only one 

 .spn -niat i/oon i-nters the egg. The sperm pem-tratr- the 

 in thr vegetative hemisphere (Figs. 20, 21 , 24). and passes io\\ard 

 tin- center \\ln-re it remains while the polar bodie- an- bcin- 

 ^i\rii ott. I 'In- -piTin nucleus tlu-n nm\ i-> to a point mar the 

 animal ]>oK- \\here the copulation of the t\\o pi.nmclei occurs 

 (Fig. 26). 



The fir^t i lr,i\a-e is unequal and iv-nlt-, in cutting off a micro- 

 mere at the animal pole. Any numhi-r ot lir-t cK-axa-f -|iindlr- 

 ha\e been observed, and they are all characteri/ed by having 

 eii;ht chromo-omi-s. and by having centnsomes which are much 

 more conspicuous than those ot the maturation spindle--. In 

 this a- in all of the subsequent early cleavages, the nuclei enter 

 into a "rest" st.i^r immediately after the completion of the 

 di\ isji in : and in-trad of forming a single vesicle, the chromosomes 



