358 ELEMENTARY BIOLOGY. [CHAP. VI. 



When young, each appears as a hill-shaped enlarge- 

 ment of the ectoderm ; when ripe, it becomes rounded 

 and very prominent. 



c. Press out an ovary : according to its stage of develop- 

 ment there will be found in it 



a. Ectoderm cells with a marked preponderance of 



the smaller form (interstitial tissue). 

 /2. Imbedded among a, one (ovicelt} which has become 



larger and clearer than the rest, and possesses a 



distinct central germinal vesicle. 



d. The ripe ovum. Conspicuous in H. viridis by its 

 green colour. It consists of a great irregularly 

 branched (amoeboid) mass of protoplasm (vitellus), in 

 which is a clear space (germinal vesicle) containing one 

 larger and a number of smaller germinal spots. 



Examine under a high power, and note 

 a. The absence of a vitelline membrane. 

 /?. The yolk granules; exceedingly large and modified to 

 form the so-called 'pseudo-cells'. Each is rounded 

 or oval, thickened on one side to form a plug- 

 shaped ingrowth and filled with a fluid contents, 

 y. The chloroplastids ; present in H. viridis; identical 

 with those found in the endoderm (Sect. 6. d. y). 



e. The segmented ovum (oosperni): composed of a large 

 number of small cells. Its thick horny capsule, 

 rough on its external surface. 



