170 T. THOMSON FLYNN 



of the embryo are arranged for. In the first of these, extending 

 over the time of cleavage and of blastocyst formation, absorp- 

 tion and exchange are performed solely by means of the tropho- 

 blast. 



^Vhen blood-vessels appear and are functional, quite a new 

 phase is inaugurated, lasting to tlie end of pregnancy, during 

 which these vessels come to the aid of the trophoblastic layer 

 in more quickly and efficiently performing the necessary 

 embryonic services. 



Grosser's (1910, p. 94) terms ' embryotrophic ' and ' haemo- 

 trophic ' could have been conveniently employed to indicate 

 these periods, but, unfortunately, his use of the word ' exclu- 

 sively ' in the definition of the latter term has made it applicable 

 alone to haemochorial placentae, and it is even douJjtful if the 

 definition would be strictly correct in their case. 



Nor does Resink's (1902) arrangement suit the case any 

 better. This author regarded the intra-uterine life of the hedge- 

 hog as falling into two well-defined periods as follows : 



(a) P r e p 1 a c e n t a 1 period, during which maternal and 

 foetal preparation for the allantoic placentation takes place ; 

 the trophospongia and ectoplacenta are formed and the 

 embryo fixed. Broadly, this period may be said to occupy the 

 earlier portion of intra-uterine existence up to the time of 

 attachment of the allantois. 



(b) E u p 1 a c e n t a 1 period, in which the allantoic attach- 

 ment is made and the placenta completed. 



The weakness of this arrangement as applied to mammals 

 generally is to be found, in ray opinion, in the extreme impor- 

 tance given to the allantoic placentation and the inclusion of 

 the yolk-sac (vascular) placentation in the first of these periods. 

 Such a scheme becomes difficult of application, particularly to 

 the M e t a t h e r i a , in which an allantoic placenta occurs, so 

 far as is known, in but one genus, various methods of tropho- 

 blastic attachment in others, in manj^ no attachment what- 

 ever, a yolk-sac placenta in all. 



It is therefore apparent that allantoic placentation is only 

 of the greatest importance in one group of mammals, the 



