In the Reptilia Braun has traced the passage of the primitive germinal cells into the testicular tubes, and I am able to confirm his observations on this point: he has not however traced their further history.
In Mammalia the evidence of the origin of the spermatospores from the germinal epithelium is not quite complete, but there can be but little doubt of its occurrence[274].
In Amphioxus Langerhans has shewn that the ova and spermatozoa are derived from similar germinal cells, which may be compared to the germinal epithelium of the Vertebrata. These cells are however segmentally arranged as separate masses (vide Vol. II. p. 54).
Bibliography.
(554) G. Balbiani. Leçons s. la génération des Vertébrés. Paris, 1879.
(555) F. M. Balfour. “On the structure and development of the Vertebrate ovary.” Quart. J. of Micr. Science, Vol. XVIII.
(556) E. van Beneden. “De la distinction originelle du tecticule et de l'ovaire, etc.” Bull. Ac. roy. belgique, Vol. XXXVII. 1874.
(557) N. Kleinenberg. “Ueb. d. Entstehung d. Eier b. Eudendrium.” Zeit. f. wiss. Zool., Vol. XXXV. 1881.
(558) H. Ludwig. “Ueb. d. Eibildung im Theirreiche.” Arbeit. a. d. zool.-zoot. Instit. Würzburg, Vol. I. 1874.
(559) C. Semper. “Das Urogenitalsystem d. Plagiostomen, etc.” Arbeit. a. d. zool.-zoot. Instit. Würzburg, Vol. II. 1875.
(560) A. Weismann. “Zur Frage nach dem Ursprung d. Geschlechtszellen bei den Hydroiden.” Zool. Anzeiger, No. 55, 1880.
Vide also O. and R. Hertwig (No. [271]), Kölliker (No. [298]), etc.
Genital ducts.
The development and evolution of the generative ducts is as yet very incompletely worked out, but even in the light of our present knowledge a comparative review of this subject brings to light features of considerable interest, and displays a fruitful field for future research.
In the Cœlenterata there are no generative ducts.
In the Hydromedusæ and Siphonophora the generative products are liberated by being dehisced directly into the surrounding medium; while in the Acraspeda, the Actinozoa and the Ctenophora, they are dehisced into parts of the gastrovascular system, and carried to the exterior through the mouth.