80 CARNEGIE INSTITUTION OF WASHINGTON. 



Development of the Male Perineum. 

 In young embryos a deep peritoneal pouch extends down to the floor of the 

 pelvis, separating the rectum from the prostatic region of the bladder. This 

 rectovesical pouch subsequently becomes obliterated to the extent of allowing 

 the rectum and prostate to come into contact, separated only by the so-called 

 Devon villier's fascia. Because of its importance in prostatic surgery. Dr. 

 M. B. Wesson has studied the embryology of this fascia and has been able to 

 demonstrate the important fact that it is not formed simply by fusion of the 

 two layers of fetal peritoneum, as had been generally supposed. It is true 

 that the layers of peritoneum do fuse, but they are absorbed and disappear 

 before the differentiation of this specific fascia occurs. This fascia arises as a 

 membranous condensation of connective tissue in the mesenchyme of the recto- 

 prostatic space. Instead of an embryonic peritoneal remnant, it is a new 

 structure, somewhat similar to the bursal formations that are found in many 

 parts of the body where there is a gliding of adjacent structures, and so notably 

 developed in the case of the bulbar fascia of the eye. Coincidently, it has 

 the important clinical function of serving as a barrier in cases of extrava- 

 sations of urine and against the spreading of infections and of malignant 

 growths. In this investigation the musculature of the perineum was carefully 

 studied, particularly the development and ultimate anatomy of the recto- 

 urethralis muscle. 



HUMAN EMBRYO OF 20 PAIRED SOMITES. 



In studying the anatomy of embryos, one usually confines his attention to 

 one organ at a time, tracing it through the various steps in its development for 

 all possible stages. But it is also well at times, and particularly in young 

 specimens, to study the embryo as a whole and carefully determine the correl- 

 ative development of all the parts. There is thus obtained a cross-section of 

 the growth process at a particular stage. We have published four such 

 studies in the Contributions to Embryology. A fifth study has just been 

 completed by Professor Carl L. Davis, of a human embiyo of 20 paired 

 somites. 



This embryo was studied with particular care because, due to its excep- 

 tionally good state of preservation, it constitutes a perfect representative of 

 this important stage in development, just after the anterior neuropore has 

 closed and when the primitive gut begins to subdivide into definitive organs. 

 This is the youngest stage in which the lung primordium has ever been 

 recognized. 



GROWTH OF THE NEGRO AND WHITE FETUS. 



The investigations upon body proportions during fetal growth have been 

 continued by Dr. A. H. Schultz and are now completed for the extremities and 

 the trunk. The head measurements have been taken, and as soon as their 

 analysis is completed it is planned to have the entire study issued as a mono- 

 graph. Preliminary accounts have already been published by Dr. Schultz, 

 in which he points out that most racial differences exist as early in fetal life 

 as the human characteristics can be recognized, and that variability in fetuses 

 is fully as high as in adults. As will be remembered from last year's report, 

 this was also found to be true in my study of the development of the external 

 ear. Dr. Schultz further shows that asymmetries do not l^egin with childhood, 

 as has been assumed, but can be recognized with certainty from the fourth 



