Segmentation of the Egg of Bdellostoma 51 



in the center of this group of filaments, is occupied- by a plug of material which Dean says 

 "may represent the granulosa, although I believe that there are grounds for regarding this 

 mass of tissue in eggs long deposited, as in part or even entirely derived from the mucous 

 elements in which the eggs have been embedded." All these structures are made clear in 

 Text-figures 1, 2, and 3. One of the sections examined by us shows the micropyle very 

 clearly, and is probably the identical section from which Text'figure 3 was drawn. 



The egg proper is composed mainly of its great mass of yolk, but at the surface there 

 is a practically yolkless layer of germinal substance thickened at the operculate end (the 

 animal pole) and traceable in sections to the vegetal pole. In eggs recently deposited, the 

 germinative protoplasm is collected into a definite hillock lying immediately below the 

 micropylar canal, as is shown in Text'figure 3. This uplifting of the germ is rendered 

 possible by the dome-shaped outpocketing in the egg-membrane. Although the germinal 

 region is thus somewhat segregated from the yolk, there is no sharp line of demarcation 

 between cytoplasm and yolk. The germinal hillock contains the egg nucleus. 



CLEAVAGE 



Since the male possesses no copulatory organs, fertiliz;ation must be presumed to be 

 external. We cannot go into the much discussed question of hermaphroditism in Bdel^ 

 lostoma. Suffice it to say that internal impregnation has never been alleged for it. Fertili' 

 zation then probably takes place at the time of spawning or immediately thereafter. Dean 

 notes that the micropylar canal is so small that only one spermatozoon can enter at a time, 

 hence he thinks that monospermy is invariable. 



Of the early development of this myxinoid, Dean wrote in his 1899 memoir as 

 follows : — 



The large egg of BdeUostorna gives . . . evidence of a partial discoidal cleavage. . . . Fertili- 

 zation takes place within the protoplasmic cap, which lies immediately below the micropylar funnel. 

 In this cap occur also the early stages of segmentation. During later divisions, the cell cap in- 

 creases in size and its margins extend downward around the [animal] end of the egg. At a late 

 stage of the blastula the cell cap occupies the entire operculate end. . . . 



At the animal pole the germinal area is distinctly lighter in color. ... Its uppermost point, 

 a minute hillock which projects toward the micropylar funnel, is the germinal cap in which the 

 earlier cleavages appear. The cell cap [m the blastula stage] . . . extends downward to nearly 

 the limits of the operculum. 



It can be said, in summary, that segmentation first appears within the protoplasmic cap, that 

 it later extends into the yolk-filled subgerminal zone, that yolk nuclei are then present and that the 

 cellular mass becomes asymmetrical as it encroaches upon and surrounds the yolk region. 



In the collection of Bdellostoma material left by Dean, all the eggs in segmentation 

 stages have been sectioned; but owing, no doubt, to long preservation of the eggs in 

 alcohol before the sectioning was done, these sections do not show details of cytological 

 structure. Our description of the cleavage is therefore confined to surface features as 

 revealed by Dean's drawings of the fresh or recently preserved eggs. 



