42 CHORD AT A. 



(60) Kupffer. "Stammverwandtschaft zwischen Ascidien u. Wirbelthieren." 

 Arch. f. micr. Anat., Vol. vi. 1870. 



(61) Giard. "Etudes critiques des travaux, etc." Archives Zool. experiment., 

 Vol. i. 1872. 



(62) C. Semper. "Ueber die Entstehung, etc." Arbeiten a. d. zool.-zoot. Institut 

 Wiirzburg, Bd. n. 1875. 



Cephalochorda, 



(63) P. Langerhans. "Z. Anatomie d. Amphioxus lanceolatus," p. 330 3. 

 Archiv /. 7iiikr. Anat., Vol. xn. 1876. 



Craniata. 



(64) F. M. Balfour. "On the structure and development of the Vertebrate Ovary." 

 Quart. J. of Micr. Science, Vol. xvm. 1878. 



(65) Th. Eimer. "Untersuchungen ii. d. Eier d. Keptilien." Archiv f. mikr. 

 Anat., Vol. vin. 1872. 



(66) Pfliiger. Die Eierstncke d. Sdugethiere u. d. Menschen. Leipzig, 1863. 



(67) J. Foulis. "On the development of the ova and structure of the ovary in 

 Man and other Mammalia." Quart. J. of Micr. Science, Vol. xvi. 1876. 



(68) J. Foulis. " The development of the ova, etc. " Journal of Anat. and Phys., 

 Vol. xin. 18789. 



(69) C. Gegenhaur. "Ueb. d. Bau u. d. Entwicklung d. Wirbelthiereier mit 

 partieller Dottertheilung." Miiller's Archil-, 1861. 



(70) Alex. Gotte. Entwicklungsgeschichte d. Vnke. Leipzig, 1875. 



(71) W. His. Untersuchungen iib. d. Ei u. d. Eientwicklung bei Knochenfischen. 

 Leipzig, 1873. 



(72) A. Kolliker. Entwicklungsgeschichte d. Menschen u. hdherer Thiere. 

 Leipzig, 1878. 



(73) J. Miiller. "Ueber d. zahhreichen Porenkanale in d. Eikapsel d. Fische." 

 Miiller's Archiv, 1854. 



(74) W. H. Kansom. "On the impregnation of the ovum in the Stickleback." 

 Pro. R. Society, Vol. vn. 1854. 



(75) C. Semper. "Das Urogenitalsystem d. Plagiostomen etc." Arbeiten a. d. 

 zool.-zoot. Instit. Wiirzburg, Vol. n. 1875. 



[Cf. Ludwig, No. 4, Ed. Van Beneden, No. i, Waldeyer No. 6, &c.] 



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 become 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 number 

 of bodies usually spoken of as test-cells make their appearance 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. 60), and Giard (No. 61), and Fol, they are also regarded as true 



