86 J. BEARD, 



system, and if they were of a nature, that no reasonable and simple 

 explanation of them could be given. 



As compared with corresponding conditions in the rabbit, or pig, 

 they are as follows. 



The fore-limb is better developed and stronger, the digits are 

 long past the webbed state, being quite separate and furnished with 

 sharp claws. 



While in the abdominal region the vertebral column exhibits no 

 advance on the condition in Eutherian embryos of the critical period, 

 in the neck and thorax it is markedly further developed than in these 

 latter; and, moreover, whereas in no other cases known to me is the 

 skeleton at the critical period other than cartilaginous, in the mar- 

 supial, as exemplified by Trichosurus, there is at this epoch bone in 

 the form of a largely ossified clavicle. A similar observation in 

 Bidelphys or Hypsiprymnus has not been recorded by Selenka, who 

 describes the skeleton of the newly-born pouch-specimens as entirely 

 cartilaginous, but in all probability the ossified clavicle was overlooked. 



In all other respects the newly -born non - placental marsupial 

 agrees with other embryos of the end of the critical period. 



The difierences just noted are all bound-up with the better degree 

 of development of the neck-region and thorax, as compared with the 

 abdomen, and are undoubtedly in correlation with the pouch-life 

 of the organism. It should be added, that this has not escaped the 

 attention of any of those, who have studied and examined new-born mar- 

 supials, indeed, it was long ago commented upon by Meckel and Owen. 



A similar, though far less accentuated, difference in the degree 

 of development between anterior and posterior regions of the trunk 

 is, indeed , noticeable in other mammalian embryos of the critical 

 period, and even in those of fishes, such as Scyllium. But in marsu- 

 pials the divergence is very pronounced. How can this be accounted 

 for? As VON Baer remarks: "was sich immer wiederholt, kann nicht 

 von Zufall oder vorübergehender Laune bedingt sein, sondern muss 

 von einer Nothwendigkeit abhängen". 



What is the necessity in this case? Non - placental marsupials 

 are destined to be born into the world in a very ill -developed, 

 though not, as usually stated, "incomplete" condition. If the term 

 "critical period", which has been applied by me to this epoch of the 

 development in other forms, have, as I have maintained, physiological 

 as well as morphological, force, it must be emphatically a critical 

 time in the life-history of a non-placental marsupial. The animal is 



