AND THOSE OF CERTAIN OTHER MAMMALIA. 93 



richly injected ; the undisturbed superficial layer, the homologue 

 of the human decidua vera, is of a uniform opaque white, the 

 injection nowhere showing its colour through its smooth unbroken 

 surface. This layer of mucous membrane abuts upon the remain- 

 ing placenta a little way within its outer rim, and upon its uterine 

 surface ; whereas in the human subject it becomes continuous with 

 it at its free edge, or even joins it a little within this on the foetal 

 aspect. In the section of the placenta we see the arborescent 

 upgrowths which carry the villi, and the downward processes of 

 maternal tissue 1 , more plainly than we see their homologues, at 

 least with the naked eye, in the lower mammals of which we 

 have been speaking. The uterine surface of the placenta is clothed 

 by a smooth continuous membrane, from which these • Decidua- 

 Fortsatze' pass downwards into it. Above the placenta, a thin 

 but coherent lamina of membrane is seen, left partly in apposition 

 with, partly divaricated from it, and joining the decidua vera at the 

 point where it impinges upon the after-birth. In its distinctness, 

 and ready separability, and coherence, it resembles the deciduous 

 serotina of the rodent ; but there can be little doubt that it does 

 not make up the whole of that layer in the macaque, but that more 

 or less of the tissue between it and the deeper strata clothing the 

 muscular walls, viz. the persistent non-deciduous serotina, would in 

 the natural order of events have been deciduous likewise. More 

 than the thin lamina may be seen to have been deciduous in the 

 natural labour of a Macacus rhesus, of which the after-birth is pre- 

 served in the College of Surgeons (Hunt. Mus., Phys. Series, 3584). 

 It is not, however, easy to say what line will exactly define the 

 limits of deciduous from those of non-deciduous serotina. For be- 

 tween the muscular coat (from which in the Simiadae and Rodents, 

 as in our own species, it is not easy to separate the mucous) and 

 the deciduous utero-placental structures a very considerable interval 

 exists, filled up with loose lamellae of tissue, the deeper of which, 

 consisting of cells with large nuclei and tapering ends, have a 

 horizontal direction, and those more directly in connexion with the 

 deciduous layers a vertical one. In this interval a considerable 



1 These are the * Decidua-Fortsatze ' of Ecker ('IconesPhysiologicae,' taf. 28. fig. 1, 

 df). They are described by Kolliker (' Entwickelungsgeschichte,' p. 145), and well 

 figured by Dr. Priestley (Lectures, p. 57. fig. 16) in the human placenta at six months, 

 after Van der Kolk. They are much less prominent in the human placenta at full 

 time. Cf. Kolliker, 1. c. pp. 143, 177, 183. 



