THE OVUM. 53 



There are some very obscure points connected with the growth 

 of the ovum of the Tunicata. When quite young the ovum is a 

 naked cell with a central nucleus containing a single large 

 nucleolus. Around it is a flat follicular epithelium enclosed in 

 a membrana propria folliculi. The follicle cells soon be- 

 come larger and give rise to an envelope round the egg of the 

 nature of a chorion. At the same time they frequently become 

 cubical or even columnar, and filled with numerous vacuoles. 



During or after the completion of the above changes a num- 

 ber of bodies usually spoken of as test-cells make their appear- 

 ance in the superficial protoplasm of the egg, which by the time 

 the egg is ripe arrange themselves in many species as a definite 

 layer round the periphery of the ovum. These bodies have 

 received their name from the opinion, now known to be erroneous 

 (Hertwig and Semper), that they eventually migrated into the 

 test or mantle of the embryo which becomes developed round 

 the ovum. By Kowalevsky (No. 58) these bodies are regarded 

 as true cells, and are believed to be formed by some of the cells 

 of the original follicular epithelium making their way into the 

 vitellus of the ovum and multiplying there. By Kupffer (No. GO), 

 and Giard (No. 61), and Fol, they are also regarded as true cells 

 but are believed to originate spontaneously in the vitellus. 

 Finally by Semper they are believed not to be cells, but to be 

 amoeboid protoplasmic bodies which are pressed out from the 

 vitellus under the stimulus of the sea-water or otherwise. 



They do not according to this author naturally appear till the ovum 

 is quite ripe, though they can be artificially produced at an earlier period 

 by the action of reagents or sea-water. When produced in the natural 

 course of things the vitellus undergoes a contraction. They are without 

 any apparent function, and play no part in the embiyonic development. 

 Semper's results are very peculiar, but owing to the careful study which 

 his paper displays they no doubt deserve attention. Further investigations 

 are however very desirable. Kowalevsky from his researches on Pyrosoma 

 (No. 59) adheres to his first opinion, though he abandons the view that 

 these cells are connected with the formation of the test. 



In the passage of the egg through the oviduct the vacuolated 

 follicle cells grow out into very peculiar long processes or villi. 

 In Ascidia canina these processes become as long as the whole 

 diameter of the vitellus (Kupffer, No. 60). 



