THE EARLY DEVELOPMENT OF THE MAESUPIALIA. 7 



early stages (ranging from the 4-celled stage to the completed 

 blastocyst), on whose examination Selenka based his account 

 of germ-layer formation in Didelphys, would thus appear to 

 have been derived from a single female.^ No wonder it is 

 impossible to reconcile his descri^jtion either with what we 

 know of germ-layer formation in the Prototheria and Eutheria 

 or with my account of the same in Dasyurus. 



My own experience with the latter has shown me that no 

 reliance whatever is to be placed on segmenting eggs or 

 blastocysts which exhibit marked retardation in their stage 

 of development as compared with others from the same 

 uterus, and also that batches of eggs or blastocysts in which 

 there is marked variation in the stage of development attained 

 should likewise be rejected. Abnormalities in the process of 

 cleavage and of blastocyst formation are by no means un- 

 common in Dasyurus, and during the earlier stages of my 

 own work I spent much time and labour on the investigation 

 of just such abnormal material as that on which Selenka, no 

 doubt unwittingly, but I feel bound to add, with an utter 

 disregard for caution, based his account of the early develop- 

 ment of Didelphys. 



I propose now, before passing to my own observations, to 

 give a short critical account of Selenka's observations, my 

 comments being enclosed in square brackets. 



The uterine ovum of Didelphys is enclosed by (1) a rela- 

 tively thin " granulosamembran," formed by the transforma- 

 tion of a layer of follicular cells [really the shell-membrane, 

 first correctly interpreted by Caldwell ('87) and formed in the 

 Fallopian tube] ; (2) a laminated Liyer of albumen, semitrans- 

 parent ; (3) a zona radiata, not always recognisable [in my 

 experience invariably distinct] . 



Cleavage begins in the uterus, is holoblastic, and at first 

 equal. A 2-celled stage is figured (Taf. xvii, fig. 3) [not 

 quite normal as regards the relations of the blastomeres], and 

 cdso a 4-celled stage [normal in appearance except for the 



1 The collection of my own early materia 1 of Dasyurus has involved 

 the slaus^hter of over seven dozen females. 



