22O ELEMENTARY BIOLOGY. [CHAP. 



Examine the peripheral portion of the youngest 

 ovisac under a high power. Look for 



ft. The vitelline membrane; transparent and well 

 denned, closely investing the ovum. 



y. The follicular epithelium ; a single layer of small 

 clear cells investing the whole ; they are slightly 

 flattened. 



8. The membrana propria ; a delicate cuticle in- 

 vesting y. 



Look for very young ovisacs. Note the rela- 

 tively small size and simple structure of the 

 ovum, as compared with the older egg-cell; 

 there is greater uniformity in size between it and 

 the cells of the follicular epithelium. 



H. The respiratory organs. Remove the branchiostegite 

 of one side and examine the gills : the functional ones 

 are 18 in number, arranged in three sets. 



1. Examine the branchial-chamber; it is bounded in front 

 by the cervical-groove, foremost among its contents 

 are 



a. The epipodites of the thoracic appendages ; seven in 

 number, all upwardly directed. The most anterior 

 one (that of the first maxillipede) is gill-less; the 

 others bear each a gill (podobranchia). 



b. The scaphognathite (Sect. B. 4. //) lying in the lower 

 anterior region of the branchial chamber, immedi- 

 ately in front of a. Examine its base of attach- 

 ment ; compare the living animal. 



2. Cut each of the gill-bearing epipodites across at its 

 base and remove thus the podobranchice as a set. Pin 

 down under water and examine any one in detail. 



