Embryonic DevclojDment in Man. 371 



of the type of those of the Peters' ovum. However, the first sec- 

 tions did not show either the cavity of the ovum or a trace of the 

 trophoblast. 



Genkkat, Description of the Ovum. 



On June 15, 1907, the block of tissue was finally divided, and 

 I am obliged to Dr. Day, Pathologist in the Chicago Laboratory 

 of the Bureau of Animal Industry, for assistance in preparing a 

 complete series of sections. The individual sections were all 7 

 microns thick ; in all they numbered 303. About three sections were 

 lost on the microtoiie; several subsequently floated off partly or en- 

 tirely from the slides during the process of staining in hematoxylin 

 and eosin, but fortunately none of the important sections containing 

 the embryonic shield were lost.^ 



The general outlines of the ovum are those of a bi-convex lens, a 

 somewhat flattened elliptical body. The ovum was found in sec- 

 tions ]^os. 63 to 235; the embryonic shield in sections 142 to 164. 

 The measurements obtained by micrometer are the following: 



Ovum (chorionic cavity-exocoelom) : Greatest length (section 

 153), 2.326 mm. Greatest width (section 153), 0.804 mm. Great- 

 est thickness (172 sections at 7 microns each), 1.204 mm. 



The trophoblast begins to show in section N^o. 47, and ends in 

 section 'No. 264. Since the chorion mesoderm begins to show in 

 section ISTo. 63 and ends in section Xo. 236, the trophoblast on one 

 pole is 16 sections or 112 microns thick, and on the other 29 sec- 

 tions, or 203 microns. In section 150 the trophoblast, towards the 

 muscularis, is a little over one millimeter thick ; towards the upper 

 surface, 0.9 mm. 



A list of the smallest human ova described, compiled from Peters' 

 and Juns-'s collection, follows : 



'The author has in the case under discussion and in a few cases of early 

 tubal pregnancies attempted to obtain complete and perfect series of ova 

 in situ by the paraffin embedding method. However, experience has taught 

 him that young human ova in situ presenting very heterogeneous tissue ele- 

 ments, including masses of maternal blood, are not best adapted for the paraffin 

 method, but should preferably be embedded in celloidin. 



