143 



of the tnesometrium, iuto isolated strands of muscle fibres; be- 

 tweeu which are uumerous closely packed cells destined, in the 

 next period, to uudergo the same changes as the cells of the al- 

 lantoidean subepithelial tissue have undergone in this. Among 

 them a few, but very few iron containing cells are still to be seen. 



b.) The further development and transformation of the tro- 

 phoblast, and the commencing disintegration of the subepithelial 

 tissue during the second period, that is to say during the vascu- 

 larisation of the trophoblast by the capillaries of the allantois. 

 As in the previous period, we may here also commence our des- 

 cription with the tissues nearest the embryo, that is to say with 

 the allantois. 



The allantois, which is entirely mesoblastic, becoraes, on its 

 attachment, intimately fused with the somatopleure covering the 

 lower surface of the cytotrophoblast. It is composed of a loose, 

 enibryonic connective tissue formed of branched, stellate cells; in 

 it are blood vessels lined by an ordinary flat endothelium. 



Very quickly this tissue, with its contained blood vessels, be- 

 comes dovetailed into the substance first of the cytotrophoblast, 

 and then of the plasnioditrophoblast, so that the embryonic blood 

 is now separated froni the maternal blood by (1) the endothelium 

 of the allantoic blood vessels, (2) the branched connective tissue 

 of the allantois, and (3) the trophoblast. For this process it will 

 be convenient to retain the ordinary expression, the penetration 

 of the allantois, but it must be remembered that the cytotropho- 

 blast takes an active part in it as well. 



The allantoic villi, as we may term these capillaries clothed 

 with connective tissue, do not penetrate the whole thickness of 

 the trophoblast, but only the lower cellular, and middle syncytial 

 portions. Consequently they rernain separated from the maternal 

 tissues by the upper portion of the trophoblast in which, as 

 previously described, glycogenesis is commencing. In their earliest 

 appearance they branch irregularly; but very soon they, together 

 with the trophoblast which covers them, become radially arranged 

 with regard to the stalk of the allantois; and at the end of this 



