DEVELOPMENT OF FLUSTRELLA HISPTDA. 447 



the body described approximates very closely to that type 

 of yolk nucleus described by Bambeke (1), Crampton (9), 

 Wallace (26), and Calkins (7) for the eggs of Pholcns 

 phalangioides, Molgula manhatteusis, Zoarces 

 viviparus, and Lumbricus respectively. 



It is not proposed in the present paper to enter into a 

 detailed consideration of the former work bearing on this 

 subject. A very complete bibliography of the yolk nucleus 

 is furnished by the papers of Jordan (15), Mortens (18), 

 Heuneguy (12), Calkins (7), Wilson (27), Bambeke (1), and 

 Crampton (9) . 



As has already been stated, the yolk nucleus of the egg of 

 Flustrella hispida is closely comparable with that type 

 described by Bambeke, Calkins, Crampton, and Wallace, but 

 on comparison with these apparently closely related bodies 

 certain points of difference may be noted. 



Bambeke (1) describes in the egg of Pholcus phalan- 

 gioides a type of yolk nucleus which corresponds closely 

 in its appearance and in its mode of growth and degeneration 

 with that occurring in Flustrella hispida. He recognises 

 four stages in the history of the yolk nucleus and nutritive 

 yolk of the egg of Pholcus: (a) the appearance of small, 

 darkly staining granules, which he believes to be of nuclear 

 origin, and which coalesce to form a crescentic structure 

 containing vacuoles, with included crystalloid bodies, and sur- 

 rounded by a clear zone ; (b) the degeneration of the yolk 

 nucleus; {c) the appearance of oil drops; {d} the formation of 

 the true 3'olk. The most important difference between Bam- 

 beke^s account of the yolk nucleus and the j^resent one is that 

 in the case of the egg of Pholcus the nucleus appears to 

 take an active part in the process of yolk formation, and that 

 the yolk nucleus itself gives rise to the vitellus by first under- 

 going a metamorphosis leading to the formation of oil 

 globules, these latter becoming resorbed by the protoplasm 

 from which the true yolk is then elaborated. Now, as has 

 already been pointed out, the presence of oil globules in the 

 case of the egg of Flustrella hispida bears but little 



VOL. 50, PART 3. NEW SEEIES. 32 



