ZOOLOGY AND BOTANY, MICROSCOPY, ETC. G23 



nervous apparatus in Lepidosteus and certain other Ichthyopsida. The 

 presence of the giant ganglion-cells in the embryo has already been 

 referred to ; in all the cases examined they occupy the same typical 

 position in the extreme dorsal or neural border of the spinal cord. 

 When a series of horizontal longitudinal sections are made these cells 

 are found to form a double row wliich reaches from the termination of 

 the hind-brain to the posterior limit of the central nervous system. 

 They are the first cells in the embryo which develope ganglionic 

 characters and they are fully developed in young embryos long before 

 the remaining cells of the nervous system become ganglionic. The cells, 

 which are multipolar, are arranged bilaterally, and there must be several 

 hundreds of them. They become all shut out of the central nervous 

 system ; these processes are either withdrawn or cut off, and their 

 poles present a curious stumpy appearance. The cells persist for a 

 long time, lying outside the cord, just over the posterior fissure. They 

 finally undergo a series of changes corresponding exactly to that 

 degeneration and death of nerve-cells which pathologists term simple 

 atrophy. It is very significant to notice that the forms in which they 

 normally occur are, without exception, oviparous. Tlie author thinks 

 that Kleiuenberg was quite right in his suspicion that the giant cells 

 described by Mayer might be analogous to certain subumbrellar ganglion- 

 cells in the larva of Lopadorliijnchus, which " introduce " the development 

 of the ventral cord ; just as in that Annelid, the development of the 

 vertebrate central nervous system would appear to have been initiated 

 by a larval nervous apparatus outside the same. 



Spermatogenesis in Mammals.*— Herr G. Niessing finds that the 

 general results of his investigations into the spermatogenesis of 

 Mammals may be thus summed up. The seminal canaliculi of sexually 

 mature Mammals contain only one kind of cell, and these give rise to 

 the spermatozoa. The cells are arranged in families which consist in 

 the resting-stage of three generations, and are disposed in columns. 

 The oldest member of a family is the stem-cell, on which follow 

 centripe tally the mother and daughter-cells. When the testis passes 

 into the active stage there is first an alteration in the form of the 

 daughter-cells. Their nucleus passes to the peripheral cell- wall. All 

 the chromatin becomes collected in the anterior half in such a way that 

 it is thickest in the equatorial plane which divides the two halves. 

 The nucleus begins to be constricted behind the equatorial plane, the 

 anterior part gradually taking on the form of the head of the 

 spermatozoon, while the part devoid of chromatin becomes colourless 

 and presents in its interior the filament ; this traverses the nuclear 

 membrane and becomes longer and longer. The altered nucleus 

 generally becomes separated in this form from the cell-body ; the 

 cylinder narrows continually around the later median-piece and produces 

 the spiral filament of this median piece, which appears shortly before 

 maturation. The tail which, by the aid of suitable reagents, may be 

 shown to consist in its anterior part of a number of very fine fibrils 

 arises from the nucleus only. The whole of the spermatozoon is 

 consequently formed from the nucleus only. 



After the metamorphosis of the daughter-cells spermatogenesis com- 

 mences in the mother-cells and then in the growing mother-cells. 

 Spermatogenesis is etiected, therefore, in three stages, and the stem-cells 



* Verhaudl. Phys. Med. Gctcll. zu WLirzburg, xxii. (1889) pp. 35-G3 (2 pis.). 



