78 J. BEABD, 



days, etc. Thus, for the tirst time the period of birth of a non- 

 placental mammal was determined, and the reasons accounting for the 

 birth taking place at such an early period of development, as guaged 

 by the standard of Eutherian mammals, were set forth at length. For 

 an account of these grounds, which appear to have been established 

 beyond question or cavil, the reader may be referred to the work 

 already cited. 



A study of Selenka's memoir certainly satisfied me completely, 

 that the birth of the opossum happened, when the embryo had reached 

 that phase of the development, which I have termed the critical period ; 

 that it did not take place before, or after, this epoch, but in the 

 phase itself. 



It was demonstrated in the "table of comparisons", that its organs 

 were then all laid down, and that, from a comparison with Raja and 

 Scyllium etc. of corresponding phases, the phrase, in 1895 first applied 

 to Baja^ could appropriately be used regarding the new-born opossum, 

 i. e. that it was "now making for the adult form". 



It was laid down, on the authority of Selenka, that the new- 

 born opossum had acquired a definite body-form, that the snout had 

 adult characters, that the anterior limbs were well-developed and 

 clawed, that Jacobson's organ was formed, that the permanent kidneys 

 were present but small, that the sex was established, that mammary 

 glands and hair-follicles were in existence, and that, thus, the animal 

 was a mammal, etc. ^). 



On one or two points Selenka's work gave no information, and 

 to obtain a knowledge of these, but, more especially, to test my con- 

 clusions as to this birth at the critical period, and, if possible, to 

 establish them on a broader and still firmer foundation I was very 

 desirous of examining a new-born non-placental mammal myself. An 

 opportunity of doing this came sooner than was expected. 



During the past winter Dr. R. Broom, who has spent several 

 years in New South Wales, and who during this period has devoted 

 much time and labour to the study of marsupial morphology, came 

 home to Edinburgh. Among his collections there is a very fine series 

 of marsupial embryos, more particularly of Trichosurus vulpecula, the 

 vulpine phalanger. 



Dr. Broom very generously placed two embryos of the latter at 

 ray disposal, allowing me to photograph and section them with a view 



1) loc. cit. pp. 22 and 74, 75. 



