VON BAER AND BSCHRICHT. 93 



Or 2. It and the maternal placenta grow together, and they 

 Lie, 



(a) in a zone round the egg. Third form. 



(b) at one end of it. Fourth form. 



These differences, however, are developed gradually, and, at 

 first, are less marked." 



The first form, described in the text of the work, is that 

 met with in the pig. It is what is now commonly termed a 

 diffused placenta; but Yon Baer, more accurate than most of 

 his successors, indicates the confinement of the placental villi 

 to the middle of the chorion — its prolonged poles remaining 

 bare — by the term " giirtelformig," zone-like. The second form 

 is that exemplified by the cow and sheep, the cotyledonary 

 placenta. The third is the carnivorous placenta, termed 

 zonular. The fourth is the placenta of man, called now-a-days 

 discoidal. 



The most important circumstance pointed out by Von Baer, 

 however, is one which has been greatly overlooked, if not 

 wholly ignored, in subsequent discussions — the fact that the 

 differences in the form of mammalian placentae are subsidiary if 

 compared with their differences in structure, more particularly 

 in regard to the extent to which a maternal element enters into 

 their composition. 



Eschricht, in the admirable memoir, " De Organis quae 

 Kespirationi et Nutritioni Foetus Mammalium inserviunt," which 

 he published in 1837, repeats the ideas of Von Baer, apparently 

 without being aware of the fact, and enlarges upon them as 

 follows (p. 30) : — 



" Restat, ut succinctam expositionem Mammalium afferamus 

 secundum varias quae in iis observantur, placentae formas. 



" A ceteris omnibus mammalibus Marsupialia et Monotre- 

 mata separanda sunt, quibus nulla est placenta. Caetera omnia 

 in duas familias dividencla, quarum alteri placenta uterina 

 caduca, alteri non caduca est. Huic Mammalia primata et 

 ungulata omnia adnumeranda sunt, inter quae Ruminantia ob 

 singularem cotyledorum formam caeteris opponi possunt. 



" In mammalibus placentam uterinam caducam habentibus 



