176 



SEVENTEENTH LECTURE. 



C- 



a~ 



'^r^SWIp 



J 





b — 





o ., 



first made in 1827, by an investigator of great merit, K. E. 

 von Baer. It appears scarcely conceivable to us ; for a sharp 

 eye sees the ovule, removed from the ruptured follicle, as a 

 white point, without a magnifying glass. The successor, 

 however, stands on the shoulders of the predecessor. 



Let us tarry for an instant at this most important of all 

 cells (Fig. 158), without which there would be no higher ani- 

 mal world. Let us remove from 

 its surface the cells, which have 

 now become cylindrical, of the 

 epithelial investment, and our at- 

 tention will be first of all attracted 

 by the thick (0.009 to °- 01 1 3 mm.), 

 resistant hyaline capsule, the so- 

 called zona pellucida or chorion (a). 

 Fig. 158.— Mature ovum of the rabbit; It is an inwardly deposited pro- 



ti, zona pellucida : /'. yolk : c, germinal 



vesicle ; ei, germinal spot. dlict of tllC SUl'l'OUnding Smaller 



cells, and, seen with higher magnifying powers, is permeated 

 by the finest radial passages, the so-called porous canals. 



The cell-body is a thick, fluid, more or less cloudy mass. 

 We perceive in it granules of albuminous matter, as well as 

 small drops of fat. In many mammalial animals, the quanti- 

 ty of the latter may become great, and the substance darker 

 and darker. This cell body is call the yolk or vitellus. 



The cell nucleus (V) attracts our attention by its elegant 

 globular form, bordered by the finest lines. It now lies con- 

 centrically; its diameter is 0.0377 to 0.0451 mm. It has 

 received the name of the germinal, or Purkinje's vesicle. 



In it, and almost always single, we finally notice the nu- 

 cleolus {d), a fat-like, glistening granule 0.0046 to 0.0068 

 mm. in size. It bears the name of the germinal, or Wagner's 

 spot, the macula germinativa. * 



* We have just become familiar with quite ordinary things provided with special 

 names. This nomenclature originated in a former epoch of embryology. Further- 

 more, the follicular walls are called theca; their epithelial lining has been denomi- 

 nated the formatio or membrana granulosa, and the cellular substance surrounding 

 the ovum the cumulus proligerus. 



