GEROULD: CAUDINA. 
59 
the inner layer of yolk substance, which reaches up into the funnel- 
shaped neck of the membrane, secretes nothing resembling a zona 
radiata, but leaves on the contrary a passage through that envelope. 
Finally, it is evident in the well-grown ovum that the zona is far 
more closely connected with the egg cytoplasm than with the follicle 
(Plate 7 , fig. 96). Its radiating fibers must be regarded, in fact, as 
processes which have arisen by secretion from the surface of the egg 
cytoplasm. 
The peculiar membrane within the ovum, which is joined to the 
follicular epithelium on one side of the egg by a funnel-shaped 
prolongation, will now be described in greater detail. In the full- 
grown ovum (Plate 6 , fig. 93 ; Plate 7 , figs. 94-100) the intravitel¬ 
line membrane consists of two parts: (1) an outer, short, tubular, 
portion and (2) an inner, very delicate, flaring portion. The former 
is connected externally with the follicle, from which it passes radially 
inward through an orifice in the zona radiata known as the micro- 
pyle, and thence inward into the yolk for a greater or less distance. 
The inner part is continuous with the outer, and in its further course 
surrounds the nucleus at a greater or less distance, probably on all 
sides. The presence of the inner part of the membrane can be 
demonstrated by treating fresh ova with one per cent acetic acid 
(Plate 7 , fig. 99), or by artificially displacing the germinative vesicle 
(Fig. 100). Sometimes, also, in sections certain radiations extend¬ 
ing inwardly from the micropyle toward the nucleus indicate its 
presence (Plate 6 , fig. 93). 
This membrane likewise exists in relation with a micropyle in the 
ovarian eggs of Cucumaria frondosa, sections of which sometimes 
afford a good idea of its appearance (Plate 8 , fig. 101). Semper’s 
(’68) figures of the ovum of Holothuria immobilis (Taf. 36, fig. 7) 
and Herouard’s of II. catensis (Planche 30, fig. 12) probably show 
the presence of this membrane in the Holothuriidae, although it is 
nowhere referred to in the text of either author. 
To sum up what has been said in regard to the nature and origin 
of the micropylar apparatus in Caudina and in Cucumaria, it may be 
stated, that it is more complicated than hitherto supposed, being 
intimately associated with a peculiar membrane, which makes its 
appearance in the yolk long before the formation of the zona radiata. 
Since the connection of this intravitelline membrane with the folli¬ 
cular cells is established before the formation of the zona begins, it 
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