THE FERTILIZATION PROCESS IN THE SNAIL. 85 



A novel opportunity to study the amphiaster presented itself in 

 an ovum which had been deposited in the sperm receptacle of a 

 snail functioning as a female during copulation (Fig. 19). In 

 this ovum the centrosome of the archiaster has presumably been 

 reduced to the typical aster-stage and is now divided to form an 

 amphiaster in which each centrosome is composed of three dis- 

 tinct centrioles similar to Fig. 25 which is a later stage and shows 

 numerous centrioles associated with the vesiculated sperm head. 

 The sperm tail is distinctly visible but is not connected with the 

 amphiaster ; the distal member of the archi-amphiaster is at the 

 periphery of the ovum. The presence of six centrioles in this 

 sperm amphiaster (Fig. 19) and of several in a more nearly 

 mature egg (Fig. 25) support the conclusions drawn from Figs, 

 n, 12, 13, 15 and 16, and other material fixed with Zenker's fluid, 

 which does not give such fine detail as the modification of Flem- 

 ming's mixture with which this unusual egg and that represented 

 in Fig. 25 were killed. This is in general agreement with the 

 work of Kostanecki and Wierzejski ('96) and Wierzejski ('06) 

 on " cross-fertilized " eggs of PJi\sa fontinalis with the exception 

 that they figure a single centriole in each centrosome. 



Mark ('81) described in Liinax a difference in the size and 

 form of the centrosomes and astral rays of the egg " archi- 

 amphiaster " of Whitney as compared with the cleavage amphi- 

 asters which parallel the relation of the sperm archiaster to the 

 sperm amphiaster which I have described for L. s. apprcssa. He 

 figures and describes a single " abnormal " egg having six " large 

 supernumerary asters with large centrosomes and very stout rays " 

 (Fig. 81 and p. 221). A similar metamorphosis of the egg archi- 

 amphiaster and of the sperm archiaster has been demonstrated by 

 Foot and Strobell ('oo, Fig. 9) for the worm Allolobophora 

 fcetida. 



A typical sperm aster has not been visible in virgin eggs until 

 the first polar globule has been extruded ; however, vesiculated 

 sperm heads occur in younger eggs (Fig. 21). In the ovum 

 shown in Fig. 14 the degenerating amphiasters must be those of 

 supernumerary sperms, while all traces of the functional sperm 

 are masked by yolk granules. Pieces of swollen, deeply stained 

 sperm tails were found in ova sectioned in the acini, but no asters 



7 



