244 ALICE THING 



The microphotographs, all of which were taken at a magni- 

 fication of 750 diameters, represent the structure of the zona 

 pellucida in eggs ranging in diameter from 0.65 mm. to 2,6 mm. 

 and from younger stages in which the zona pellucida measures 

 only Iju up to a stage where it is 17ju in thickness. I do not know 

 if this last measurement may approximate the maximum thick- 

 ness of the zona since I have no measurements from larger eggs. 

 Mile. Loyez states that in reptiles the zona is verj^ thin upon 

 completion of development. Since it is extremely difficult in 

 microphotography to focus upon an entire field unless that field 

 is perfectly flat in all its parts some portions 'of the figures are not 

 sharply defined. The endeavor has always been to focus upon 

 the most mportant part of the section. 



OBSERVATIONS 



The epithelium 



When the oocyte has reached the size two or three times, at a 

 rough estimate, that of the oogonium from which it originated 

 it is surrounded by a flattened epithelium which remains of one 

 layer throughout the course of development. With the gradual 

 growth of the oocyte the epithelial cells take on a definite pris- 

 matic shape and increase in height in the axis perpendicular to 

 the surface of the egg until this axis may become as long as the 

 transverse. The transverse axis appears the longer, however, 

 in the majority of cases especially in the later stages herein de- 

 scribed. Upon cross sections through the epithehal layer of 

 oocytes less than 1 mm. in diameter the nuclei of the epithelial 

 cells are seen to be rather widely spaced (fig. 2, ep.) while in older 

 stages, because of reduction in size of the nuclei and in content 

 of the cytoplasm, the arrangement is more compact (figs. 3, 7, 9, 

 10, 11, ep.). Occasional mitoses prove that to accommodate 

 the increasing volume of the egg the epithelium extends itself by 

 divisions of its constituent cells. In eggs much larger than those 

 figured very numerous mitoses occur. The epithelial cells are 

 sharply marked off from one another by intercellular channels 

 filled with intercellular substance. Unfortunately this does not 



