3 1 Jan., 1909.] JUcmcnts of Animal Physiology. 31 



UMBILICAL CORD. The shapc and attachment of the placenta varies in 

 different species of mammals ; in some the finger-shaped processes or villi 

 are simple ajid when the young is born are detached from the uterine wall 

 without producing any tearing. In such cases the placenta comes away 

 without any portion of the uterus being attached to it — such placentas are 

 termed non-deciduous. In others the villi are so locked in the uterine 

 tissue that when the young is born the \illi drag with them pieces of the 

 inner wall of the uterus and leave what may be termed an open sore at the 

 site of the placental attachment. Such cases are called deciduate. We 

 may classify placentation roughly as follows : — 



rMeta-discoidal — the villi are at first scattered, but are collected into a 



-p. . 1 I disc — man and monkey. 



I Discoidal — the villi are restricted to a cake-like disc— rodents. 



(Zoiuiiy — the villi are restricted to a beltdike V)and— carnivores, elephant. 



T.T 1-1 ( Cotyledonary — villi in patches — ruminants. 



Nondeciduous \t\-c^ -n- ..4. 1 ■ 1 



\ Diiiuse — villi scattered — pig, horse. 



As development proceeds the cells of the embryo increase continually 

 by subdivision and the organs approach nearer and nearer to the condition 

 that thev present at birth. One extraordinary fact about embryonic de- 

 velopment is that a number of stages are traversed which are singularly 

 reminiscent of stages in the evolutionary ascent of the race. Thus all 

 mammalian and bird embrvos at one period of growth show clefts in the 

 throat like the gills of a fish, and the human embryo at one time possesses 

 a tail. 



From the moment of entry of the fertilized ovum the uterus begins to 

 change. Instead of the metoestrum a long series of important alterations 

 takes place too complex for full description here. The uterus grows 

 parallel with the growth of the embr\o and the membranous bag of the 

 latter, its muscular walls increase enormously in thickness and in the size 

 and power of the constituent muscular cells, and, where the placenta is to 

 be formed, an alteration in the inner wall is effected. These important 

 changes in the uterus are the outcome of hormone stimulation. The de- 

 veloping ovum unquestionably furnishes one set of hormones, but these 

 would apparently be useless unless backed up bv hormones arising from 

 the O'vary and probably from one particular part of the ovary called the 

 •corpus luteum (Fig. 63). The corpus luteum is produced from a burst 

 follicle and was originally thought to be only a lump of scar tissue 

 but its secretorv importance has been proved beyond doubt for if it be 

 ■destroyed or if it fail to develop pregnancy comes to a premature end. 

 What exactly determines the onset of parturition or labour has not been 

 •discovered. The muscular walls of the uterus contract powerfully in a 

 series of " pains." Thanks to the membranes containing the watery fluid, 

 the narrow neck of the uterus can be dilated without injurv being done to 

 the head or fore Fimbs of the foetus. The pains continue until the mem- 

 branes burst and the young, after undergoing a characteristic rotation, is 

 driven through the vagina ; after a variable interval the membranes and 

 placenta are ejected by further uterine contraction. Once the placenta has 

 been detacbed from the uterine wall the young animal can no longer get 

 its oxygen from the maternal blood ; slight asphyxia therefore ensues and 

 through the stimulation of the respiratory centres by the asphyxial blood 

 the first breath is taken. Coincidentally with this a change takes place in 

 the heart .so that the right ventricle which in the foetus sent its blood into 

 the aorta now drives blood into the lungs through the pulmonary artery 

 .and the condition of the circulation present in the adult is established. 



