518 MR. F. E. BEDDARD ON THE DEVELOPMENT AND [D CC. 7 



migrated into the interior ; the principal evidence is in favour of the 

 latter supposition. 



The central mass itself is, as already stated, mainly occupied by a 

 mass of yolk-spherules ; these are deeply stained by the colouring- 

 reagent. Among the mass of yolk-spherules are numerous cells 

 which are more scattered in later stages ; many of them are in 

 course of degeneration, as evinced by the characters of the nuclei. 

 In my former paper I have figured {loc. cit. pi. xxix. figs. 9-20) a 

 number of such cells, and need not refer to the matter again here. 



In no case could I observe the faintest trace of a germinal vesicle, 

 nor does any one of the cells found in the interior of the body show 

 any preponderance in size, or difference of any kind from the rest. 

 The vascular layer is highly developed in this stage, in accord with 

 the rapid growth of the follicular cells. 



Outside the vascular layer is the secondary follicle-layer, which 

 has much dwindled in importance, and has a still closer resemblance 

 to stroma than it had in the earlier stages. 



Contents of the Ovary of Ceratodus. 



The ovary of Ceratodus, like that of Protopterus, contains, 

 besides the ordinary ova which follow a normal course of develop- 

 ment, other remarkable structures similar to those of Protopterus. 

 The normal kind of ova, which are by far the most abundant, repre- 

 sent a single cell, and agree in most details of structure with the ova 

 of Amphibia and the corresponding ova in Protopterus. The mature 

 ova are filled with rounded yolk-spherules approximately of equal 

 size, and entirely unstained by prolonged immersion in borax 

 carmine ; the periphery of the ovum is occupied by a delicate layer 

 of egg-protoplasm in which pigment-granules are imbedded ; the 

 egg is covered by only a single membrane, which is moderately thick 

 ■with radial pores ; the follicular epithelium is a single layer of 

 flattened cells, of which the nuclei alone are obvious in my preparations. 

 Some ova belonging to this stage are figured by Ayers in his 

 paper. In rather younger ova there is a ball of protoplasm in 

 the centre surrounding the germinal vesicle, and not yet invaded 

 by the formation of yolk ; the germinal vesicle has a peripheral row 

 of germinal spots as in Protopterus. The formation of the yolk 

 appears also to be on the whole very similar to the process described 

 m Protopterus. Aggregations of yolk-granules make their appear- 

 ance throughout the ovum, but do not seem to be confined at first 

 to the peripheral layers. In the younger stages the yolk-particles 

 are deeply stained by borax carmine, but not in the adult ova. 



I have already stated in the remarks introductory to this paper 

 that, as regards my specimens, Ceratodus differs from Protopterus in 

 the extreme rarity of those bodies which are apparently formed by a 

 fusion of a number of distinct cells. So very rare are these structures 

 that after a diligent search I only succeeded in discovering a single 

 case which could be in all probability referred to Stage I. in the 

 developmental history of Protopterus (see p. iJ08). Certain other 



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