The Birth-period of Trichosurus vulpecula, 37 



expelled from the maternal uterus, lifted up by the mouth of the 

 mother, and applied to the teat^). 



The young one must have at least strength enough to grasp the 

 teat with its mouth, the anterior region of the body must be suffici- 

 ently developed to support the weight of the abdomen ^), when hanging 

 on the teat. But what explains the ossification of the clavicle? Ac- 

 cording to Dr. Broom, who has had unusual opportunities of observing 

 newly-born marsupials of several genera , the young animal , when 

 removed from the teat, makes constant "clawing" movements with its 

 fore-limbs. These undoubtedly fulfil two purposes , they enable the 

 foetus to get possession of the teat in the first instance, when applied 

 to it by the mother, and they enable it to retain its hold once there. 



The possibility of such movements of the limbs postulates a certain 

 degree of development of muscles and skeleton. The effecting of these 

 movements, then, requires the ossification of the clavicle and the ad- 

 vanced development of the vertebral column and neural arches. 



Thus, all the points of difference are such as are easily explicable, 

 as due to the conditions, for which the embryo must be prepared, by 

 the time the birth-period arrives. They are all special adaptations 

 to the necessities of the pouch-life, and they do not in any way nega- 

 tive the existence of the critical period in the life-history of a mar- 

 supial, or its coincidence in non-placental forms with the birth-period. 

 Although, as a general, nay, almost universal, rule, the degree of 

 development at the critical period is about the same ^) in various 

 vertebrates, this is brought about by extrinsic, rather than by intrinsic, 

 circumstances, and it would not be very incorrect to describe it as 

 more a coincidence than a necessary condition. 



What really determines the critical period, as was insisted a year 

 ago, is the state of the embryo as a whole, and not the histological 

 condition of some one or more organs of secondary importance. 



The critical period in a morphological sense is that epoch of the 

 development, when all the parts of the organism are first present as 



1) Owen, R., Marsupialia, in: Todd's Cyclopaedia, 1839 — 1847, 

 p. 322—323. 



2) It has been pointed out to me , that there is a decided ad- 

 vantage in keeping down the weight of the abdomen in the newly- 

 born foetus. 



3) So far I have always carefully avoided saying, that it was 

 exactly the same, and on a former occasion the word "about" was 

 employed as intentionally as in the above passage. 



