Embryonic Development in Man. 3G7 



of the ovnni. It is composed of higli cylindrical cells, some of which 

 show mitoses. The extremities of the crescent are continnous with 

 the closed amnion, which is composed of one layer of flat cells. 

 The cavity formed by the embryonic shield and the amnion in trans- 

 verse section has a lenticular shape, and it is filled with a finely 

 granular coagulated mass. Externally the amnion and embryonic 

 shield are surrounded by a thin layer of mesoblast, . . . which 

 separates them from the yolk sac. The latter can only be seen dis- 

 tinctly in twelve sections. The yolk sac shows a simple layer of 

 flat entoderm cells. Toward the embryonic shield this ento- 

 blast rests directly upon the mesoblast which separates the ecto- 

 dermal shield from the yolk sac; toward the other side the ento- 

 blast is strongly curved, and quite a distance removed from the 

 mesoblast. The yolk sac, as a whole, has a somewhat hemispherical 

 shape, with its base directed towards the embryonic shield. 



"The mesoblast, as already mentioned, extends from a broad base 

 attached to the chorion towards the 'Embryonalanlage,' and forms 

 at its basal side the allantoic stalk. The latter is composed of 

 mesoblast cells only, and shows no trace of an epithelial-lined duct, 

 . . . Thei'e are no distinct vessels. However, one sees here and 

 there at the periphery of the mesoblast accumulations of cells, 

 occasionally arranged in a circular manner, with a lumen in the 

 middle. Similar formations are seen on the lateral portions of 

 the allantoic stalk. '^I am unable to decide whether we are dealing 

 with the first 'Gefiissanlagen.' The circular rings do not show 

 in their lumen anything like blood corpuscles. ISTumerous mitoses 

 are seen in the embryonic shield and in the allantoic stalk." 



History of the Case FROisr Which the Ovum Came.^ 

 Our own ovum, while not coining up fully to the most ideal re- 

 quirements, approaches them so closely that it must indeed be a 



*Tlie first more extensiA-e cominuuicatiou oonceniing our specimen was 

 made liefore the Section of Embryology of the Seventh International Con- 

 gress of Zoology, held in Boston, in August, 1907. It was then the privilege 

 of the author to demonstrate the sections to such well-known European and 

 American embryologists as Professors A. A. W. Hubrecht, F. W. van Wijhe, 



