THE FERTILIZATION PROCESS IN THE SNAIL. 8 1 



themselves into an equatorial plate. Few or no yolk granules 

 pass into either polocyte and one or both polocytes may remain 

 attached to the egg for several cleavages, or even until the gastrula 

 stage. Holmes ('oo) found that in Planorbis the polar bodies 

 were retained until the gastrula stage was reached. 



Sections show that the chromosomes of the first polar body 

 normally arrange themselves in an irregular equatorial plate, and 

 in some instances definite monocentric mitoses, such as are shown 

 in Figs. 34 and 59 occur. Griffin ('99) found that in the 

 Echiuroid Thalasscnia the first polocyte normally divides syn- 

 chronously with the formation of the second, the chromosomes 

 forming double elements before dividing and in general behaving 

 as do the egg chromosomes. He shows that rarely by the time 

 the second polocyte was extruded the chromosomes of the first 

 polocyte had completed mitosis to the telophase without the 

 cytoplasm having divided. 



In L. s. appressa the second polocyte forms about four distinct 

 vesicles synchronously with the formation of the egg nucleus 

 (Figs. 34, 40). These vesicles disappear some time before those 

 of the egg fuse, leaving a few more or less deeply staining chro- 

 momers which become larger, more numerous and stain more 

 deeply as the first cleavage nucleus is formed (Figs. 27, 32, 35, 

 42). 



VIII. INSEMINATION. 



At first thought one would be led to assume that self-fertiliza- 

 tion and polyspermy occur under such conditions as have been 

 shown to exist in the hermaphrodite apparatus of L. s. appressa 

 unless some barrier, such as Morgan ('04, '05, '23) found in 

 dona existed. Investigation shows that this is the case; the 

 hyaline membrane acting as a barrier to the entrance of the 

 spermatozoa until an area of the egg is freed from it (Figs. I, 12, 

 46). 



A. Conoid and Round-head Spermatozoa. 



Ova from the hermaphrodite duct contain only round-head 

 sperms (Figs. 6, 15, 16) and are usually surrounded by the same 

 type of spermatozoa (Fig. 10), while those ova from the acini 

 of the ovotestis usually contain only conoid sperm heads (Figs. 



