THE CLOACA IN HUMAN EMBRYOS / 



The questions that appeal to the writer as doubtful ones may 

 be expressed as follows: 



1. Does the cloaca proper ever open to the outside? 



2. Does the cloacal membrane arise directly by displacement 

 of applied layers of ecto- and entoderm in the primitive streak 

 or is it formed in situ through disappearance of the intervening 

 mesoderm? 



3. In what manner is the cloacal division effected? 



4. What is the anlage for the bladder? 



5. Can the urinary function of the mesonephros assumed by 

 Nagel be demonstrated? 



MATERIAL 



The study of the cloacal region was done in part by working 

 out the relations in serial sections, and in part through reconstruc- 

 tion. Thirty embryos in all were examined; reconstruction em- 

 ployed in thirteen, of which six stages will be presented. The 

 material, with the exception of these six , is given entirely in the 

 tabulation. The serial number, used throughout this paper, 

 refers to the age of the embryo based on the development of the 

 urogenital tract and with no particular reference to its length. 

 It is interesting to note that with the exception of nos. 4, 11, 13, 

 and 21, the development of the tract has proved to be an excel- 

 lent check on the determination of the age by the greatest length 

 method. The Mall number refers to the catalogue number of 

 the collection of human embryos at Johns Hopkins University 

 and the section thickness (indicated in microns) ; the section direc- 

 tion (+ for transverse, = for sagittal, and || for coronal; and the 

 condition (/ for fair and g for good) are recorded in this manner 

 in the Mall catalogue. The modelling of the embryo is indicated 

 by the magnification of the same, and where the model is presented 

 in this paper it is designated by a capital letter. The brief normen- 

 tafel shows the major points of difference in the development of 

 the cloacal region. The embryo 30 is the Piper II embryo of the 

 collection, and will be presented later in connection with the origin 

 of the bulbovestibular glands. This embryo and also no. 20 have 



