116 J. p. HILL. 



Amongst the Marsupials the zoua is quite thin (about "0016 

 mm. in Dasyurus), presumptive evidence that it was also thin 

 in the ancestral stock from which the Meta- and Eutheria 

 divei-ged, whilst amongst the Eutheria themselves the zona, 

 as Robinson ('03) has pointed out, is not only of very varying 

 thickness, but persists round the ovum for a very varying 

 period in different species. It appears to be thinnest in the 

 mouse ('001 mm.), in most Eutheria it is considerably thicker 

 ('01 mm., but, dog, rabbit, deer), whilst in Cavia it reaches 

 a thickness of as much as '02 mm. In those forms in which 

 the blastocyst early becomes embedded in, or attached to, the 

 mucosa, the zoua naturally disappears early. In the rat, 

 mouse and guinea-pig it disappears before the blastocyst is 

 formed. Hubrecht failed to find it in the 2-celled e^g of 

 Tupaia, and it was already absent in the 4-celled stage of 

 Macacus nemestrinus, discovered by Selenka and de- 

 scribed by Hubrecht. On the other hand, it may persist for 

 a much longer period, up to the time of appearance of the 

 primitive streak (rabbit, dog, ferret). These facts suffi- 

 ciently demonstrate the variability of the zona in the Eutherian 

 series, and its early disappearance in certain forms before the 

 completion of the blastocyst stage shows that it can have no 

 supporting function in regard to that. 



Postulating, then, the disappearance of the shell-membrane 

 and the presence of a relatively thin, non-resistant zona (with 

 perhaps a layer of albumen) round the minute yolk-poor ovum 

 of the primitive Eutherian, and remembering that the ovum 

 starts with certain inherited tendencies, the most immediate 

 and pressing of which is to produce a blastocyst comprising 

 two differentiated groups of cells, the problem is how, in the 

 absence of the old supporting sphere constituted by the egg- 

 envelopes, can such a vesicular stage be most easily aud 

 most expeditiously attained ? The Eutherian solution as we see 

 it in operation to-day is really a very simple one, and withal a 

 noteworthy instance of adaptation in cleavage (Lillie, '99). 

 In the absence of any firm supporting membrane round the 

 egg, and the consequent impossibility of the blastonieres pro- 



