356 Proceedings of the Royal Physical Society. 



with abundant yolk, and the process of cell division is con- 

 fined to a protoplasmic germinal disc, and does not include 

 the yolk. The early developmental stages of the Monotre- 

 mata therefore are necessarily similar to the corresponding 

 stages in the development of the Sauropsida, and so far dis- 

 similar to the other groups of the Mammalia. This fact has 

 'perhaps, as has been suggested, some bearing upon the origin 

 of the Mammalia, as tending to show that they have been 

 the offspring of Sauropsidan-like ancestors, and have not 

 been derived, as has hitherto been believed, from a group 

 more closely resembling the existing Amphibia, inasmuch as 

 the ova of the latter group undergo total segmentation. It 

 is obvious, however, that this deduction may be questioned, 

 and no doubt Mr Caldwell will be able to furnish more 

 evidence on the point from a study of the whole course of 

 development of the Monotremata. 



With regard to the partial segmentation of the ova of the 

 Monotremata, it appears to me to be just to remind the 

 Society that my friend Mr E. B. Poulton had previously 

 described the structure of the ovarian ovum of Omithor- 

 hynchus, and to some extent of Echidna, and that his results 

 had rendered it extremely probable " that segmentation is 

 unequal, perhaps partial." This does not, of course, detract 

 from the merit of Mr Caldwell's discovery, who independently 

 proved to be a certainty what Mr Poulton could naturally 

 only regard as a probability. 



In a very interesting paper recently published in the 

 Quarterly Journal of Microscopiccd Science} and already cited, 

 Mr E. B, Poulton has described the structure of the ovarian 

 ovum of Ornithorhynch'us and Echidna, more particularly the 

 former. The result of his investigations has been to show 

 that in the Monotremata the ovum occupies the whole of the 

 follicle, and that the epithelium of the follicle remains a 

 single layer during the whole time that the ovum is enclosed 

 within it. This result is clearly a very important one, inas- 

 much as it indicates a close resemblance to the conditions 

 met with in the ovary of the lower Vertebrata, and a con- 

 siderable difference from the same structures in the Mam- 



^ Quart. Jour. Micr. Sci., January 1884. 



