688 PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 



globule passes in later stages under the embryo, and for some time moves freely; but 

 about the fifth day, when the blastopore closes, it becomes fixed, generally at the point 

 coinciding with the vegetal pole. It now exhibits a thick layer of protoplasm, which 

 becomes vacuolated in a complex manner, and gives origin to numerous nuclear structures 

 as well as pigment-spots (PL XI. figs. 12, 13). In certain cases (gurnard) the peri- 

 blast was observed to bend in from the blastodermic layers, and carry the oil-globule 

 with it at its margin.* 



Perivitelline Space. 



This space is generally very distinct, and contains a transparent fluid, usually said to be 

 water, which enters the ovum after fertilisation. In an undetermined species the space 

 is enormously enlarged (PI. XIII. fig. 3). Eeichert, however, very lately observed that 

 under the action of nitric acid it exhibited whitish flakes (No. 134, p. 463). Ransom again 

 states that when the funnel of the micropyle is withdrawn from the discus proligerus (in 

 Gastrosteus), water enters " to fill the breathing chamber." This view was questioned by 

 one of us in a short paper read at the British Association in 1885 (No. 122), the following 

 statement being made : — " That a certain amount of water finds access to this space is 

 possible, but in stained sections the fluid filling the chamber often appears coagulated and 

 faintly stained, thus indicating the presence of minute protoplasmic particles. It would 

 appear to be, therefore, a dilute plasma." Raffaele has recently stated that the fluid 

 is albuminous (No. 125a). In the gurnard various granular bodies, probably portions of 

 protoplasm of a circular form, have been seen. It is possible that these agree with the 

 segmenting corpuscles of Ryder and the expulsive bodies of Raffaele. t 



III. Extrusion and Deposition. 



The ova when ripe either pass directly into the body-cavity from the ovaries and out 

 by an external pore, as in the Salmonidse and Anguilla, or they pass to the posterior end 

 of the ovary as they become mature, and thence by an oviducal aperture to the exterior. 

 The latter is the more prevalent mode, and it presents two types according as the act of 

 deposition is very rapid, as in the Cottoids, Discoboli, and certain Blennies ; or it is slower, 

 as in the Gadidae, and may be even prolonged, as appears to be the case with Trigla. 



The difficulty of ascertaining the real facts in regard to oviposition is apparent when 

 it is remembered that, about even so familiar a form as the salmon, opinion has been 

 up till comparatively recent times divided; the fishermen being of opinion that the 

 process is gradual, and may occupy many days, whereas there is much evidence to show 

 that the ova are discharged all at once, or very rapidly. In stripping a ripe female the 



* J. A. Ryder (Atner. Nat., vol. xx. p. 987) states that the periblast is hypoblastic, and that the only source of 

 the nuclei in the pigment-cells of the oil-drop must be periblast; therefore these cells are hypoblastic in origin. 

 Kingsley and Conn severally observe (op. cit., p. 188) that possibly the ova of all the Gadidse have one or more 

 conspicuous oil-globules in the deutoplasm. 



t Op cit., p. 16. He also thinks the perivitelline space has a phylogenetic significance. 



