1886.] OVUM OF LEPIDOSIREN. 273 



Concerning the origin of the ova I have no observations to offer. 

 I shall therefore at once proceed to the discussion of the different 

 parts of the ovum and its follicular epithelium. 



II. Egg-membranes. 



The question of the origin of the different egg-membranes and 

 their homologies throughout the Vertebrate series has always been 

 one of interest ; but the opinions held with regard to these points 

 differ very much. Balfour considers it tolerably certain ^ that in 

 most Vertebrata there are two egg-membranes present — (1) an outer 

 structureless vitelline membrane, and (2) an inner perpendicularly 

 striate zona radiata, both of which are formed as a differentiation of 

 the egg-protoplasm. Of these two membranes the vitelline is obviously 

 the first to appear ; the zona radiata subsequently makes its appear- 

 ance, but is reabsorbed into the ovum some time before the latter 

 is mature ; the vitelline membrane persists for a longer period, but 

 eventually disappears. In the youngest ova of Protopterus I could 

 detect no trace of any membrane ; as development proceeds a very 

 delicate homogeneous membrane encircles the ovum. This membrane 

 probably corresponds to the vitelline membrane, but I have no 

 exact observations to offer on its origin ; judging from analogy it is 

 a product of the egg-protoplasm, which at this stage (Plate XXIX. 

 fig. 8) is of uniform granular ajipearance. At the same time the 

 membrane was not at all firmly adherent to the ovum, but seemed 

 to have a much more intimate relation to the follicular epithelium ; 

 in sections that were less perfect than others this membrane was 

 detached froqi the surface of the ovum. 



In more mature ova — of the stage represented in Plate XXVIII. 

 fig. 1 — there was present, in addition to the vitelline membrane, a 

 thick membrane lying beneath it, and evidently formed as a 

 speciahzation of the peripheral layer of the ovum. For the most 

 part this membrane was as distinct from the subjacent protoplasm 

 of the ovum as is shown in Plate XXIX. fig. 2. Here and there it 

 appeared to pass gradually and without any break into the substance 

 of the protoplasm. This membrane shows radial striations (Plate 

 XXIX. fig. 2), and appears to me in all probability to correspond to 

 the zona radiata of other Vertebrata. The extreme thinness of the 

 vitelline membrane as compared with the zona radiata is illustrated 

 in Plate XXIX. fig. 2. In this stage there was no trace of any 

 formation of yolk except that the egg-protoplasm has taken on a 

 reticulate arrangement (see Plate XXIX. fig. 2) in preparation for 

 the yolk-formation soon to commence. 



In the next stage, in which yolk-formation has already commenced, 

 the conditions of the egg- membranes are slightly different, the dif- 

 ference principally relatmg to the outer vitelline membrane. The 

 vitelline membrane in this stage is very markedly thicker than in 

 the previous stage ; it is also much more intimately connected with 

 the follicular epithelium than with the zona radiata. The latter 



^ Comp. Embr. vol. i. p. 60. 



