228 PHYSIOLOGY CHAP, vi 



of these writers, and in showing by his own investigations that 

 the foetus does not regulate the development of the placenta 

 or the placenta that of the foetus, but that all the different parts 

 of the ovum (foetus, placenta, membranes, umbilical cord, amniotic 

 fluid) are normally mutually related as regards their development, 

 in the sense that when one part of the ovum is more highly 

 developed, all the others will be proportionately so. From his 

 own observations of the relations between the weight of the 

 placenta and that of the foetus, Sfameni came to the conclusion 

 that the heavier placenta supplies the corresponding foetus with 

 an amount of nutritive material smaller in proportion than that 

 supplied by the smaller placenta. 



The placenta has 'a spongy consistence and is dark grey in 

 colour. The foetal surface is concave, smooth, and covered with a 

 thin transparent membrane called the amnion \ in ifc are found 

 the ramifications of the placental vessels converging towards the 

 umbilical cord, which is excentrically attached (Fig. 91). The 

 uterine surface is convex, irregular, and sanguineous owing to the 

 lacerations caused by detachment, and is divided into lobes or 

 cotyledons (Fig. 92). On its circumference the placenta is 

 continuous with the parietal decidua. 



III. The way in which the embryonic membranes arise in the 

 higher animals and in man differs from that which we know to be 

 the case in the sauropsida and lower mammals, in which it is 

 accomplished by means of a very well-known and somewhat simple 

 mechanism. 



At an early stage of development furrows (limiting sulci of 



FIG. 93. Diagrams showing the formation of the embryonic adjuncts in a mammal. A, B, C, 

 transverse sections through axis of the embryonic area. D, E, F, G, H, sagittal median sections. 

 The ectoderm is coloured red, the mesoderm black, the endoderm blue. The body of the 

 embryo with the exception of the surface is indicated by a uniform grey. The cephalic 

 extremity of the embryo is towards the right, a, amnion ; at, embryonic area ; ul, allantois ; 

 ap, amniotic fold ; c, body of the embryo ; ce, external -coelom ; ch, chorion ; chp, cliorion 

 in the region where the placenta will be developed ; i, intestine ; o, point of junction of the 

 amniotic folds ; s, serous covering ; vo, umbilical vesicle ; *, mouth ; **, anus. 



A, the embryonic area is differentiated ; the endoderm is confined to the upper segment 

 of the ovum. 



B, the endoderm has spread to the lower pole ; the mesoderm has appeared in the region 

 of the embryonic area. 



(7, the mesoderm has spread to the equator of the ovum and the coelom has formed within 

 it, both in the embryonic area and outside it ; the amniotic folds rise ; the umbilical vesicle 

 begins to be distinguishable. 



D, the amniotic folds have grown, but have not yet met ; the intestine communicates freely 

 with the umbilical vesicle. 



E, the amniotic folds have met, the amniotic cavity has closed ; the umbilical vesicle has 

 shrunk and, owing to the extension of the coelom, has become more detached from the ecto- 

 derm ; the allantois is roughly outlined. 



F, the serous covering is complete ; it has become detached from the amnion and umbilical 

 vesicle and has developed villi ; the umbilical vesicle in its shrunken state is free in the 

 coelom ; the alimentary canal has opened at its two extremities, forming the mouth and the 

 anus ; the allantois has developed. 



G, the umbilical vesicle has become still smaller and its peduncle narrower ; the allantois 

 with its vessels has joined the serous covering, which becomes the cliorion, and its mesoderm 

 penetrates into the chorionic villi. 



H, the amniotic cavity has become larger and the amnion is about to adhere to the inner side 

 of the chorion ; the umbilical vesicle and the allantois are much smaller and their peduncles 

 have become long and thin; the villi of the chorion have extended to the whole membrane 

 and have grown, especially in the zone which corresponds to the insertion of the allantois, 

 where the placenta will develop later on. 



