540 



Scientific Proceedings, Royal Dublin Society. 



Haacke (9) and Caldwell have recently demonstrated that 

 the previously known [cf. W. B. Spencer, " The Eggs of Mono- 

 tremes," Nature, xxxi., p. 132, December 11, 1884) but discredited 

 fact that the Monotremata are oviparous, and that the eggs are per- 

 fectly comparable with those of reptiles. Thus Balfour's deduc- 

 tion, from purely embryological data, that the mammalian ovum 

 originally possessed a large quantity of food-yolk, has been tho- 

 roughly verified. 



Yan Beneden has shown that this " blastopore " is a marked 

 feature in the ovum of a rabbit. Balfour (2, fig. 134a, p. 178) 

 and Allen Thomson (in Quain's Ana- 

 tomy, ii., 1882, fig. 631a, p. 744) have 

 reproduced the stage just before it is 

 closed over, and thus do not show it at 

 its greatest development. Yan Beneden 

 and Ch. Julin (6) figure a segmented 

 ovum of the great horse-shoe bat in a 

 very similar stage (woodcut, fig. 1), the 

 chief differences being that in the for- 

 mer the outer cells are quadrate and 

 not somewhat flattened ; the inner mass 

 entirely fills up the cavity of the blas- 

 todermic vesicle, instead of there being a considerable cavity as 

 in the bat. In the former, the cells of the inner mass are well 

 defined, whereas in the latter " it is impossible to distinguish the 

 limits of the cells;" and, lastly, "the break in the continuity of 

 the ectoderm, occupied by several ' endodermic ' cells, appears to 

 be homologous with the ' blastopore ' of the rabbit ; it is here of 

 much greater extent and persists longer. In the rabbit it nearly 

 always closes before the blastodermic cavity commences to form." 



In the earliest stage of the mole, figured by Heape (10), the size 

 of this " blastopore " and the general appearance of the ovum suffi- 

 ciently resemble the last stage of the rabbit's " blastopore " men- 

 tioned above. 



Dr. A. Fraser informs me he has not yet satisfied himself as to 

 the presence of such a " blastopore " in the rat or the mouse. 



The accompanying diagrams (figs. 2-5) may render this view 

 more clear. Fig. 2 represents the ovum of a hypothetical primi- 

 tive mammal (the monotreme's ovum is doubtless very similar to 



;. I.— Blastodermic vesicle of the 

 great horse-shoe bat (after van 

 Beneden and Julin). 



