. ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 21 



which the posterior ends of the uterine segments of the Miillerian ducts, 

 and the entire vaginal segments of the same, never become free, except 

 in so far as the for ward ly projecting vaginal caeca may be said to have 

 become free from the original tissue of the cord. 



Pouch of Echidna.* — Prof. E. Semon admits, after Euge's criticism, 

 that he was mistaken in speaking of the rudiment of the pouch in the 

 Tery young Echidna as paired. It is a single depression. He also refers 

 to his observation of a female Ornithorhynchus at the height of lactation 

 which did not show the slightest hint of a pouch. It has been supposed 

 by some that there might be a temporary depression. 



Young of Ornithorhynchus.f — -V. Sixta communicates the field-notes 

 of A. Topic on the breeding of the duckmole. The mouth of the burrow 

 is below water-level ; the nest chamber (as large as a platter and as high 

 as a loaf) is above high-water mark, and lined with hair from the backs 

 of both male and female. In giving milk the mother lay on her back, 

 and her two young tapped with their bills about the sieve-like openings 

 of the glands. The milk ran into a median groove on the skin formed 

 by the longitudinal musculature, and was thence lapped up. The young 

 remain in the nest till they attain a size of 12 cm. ; when 20 cm. in size, 

 4hey venture with the mother into the water. 



b. Histolog-y. 



Structure of the Cell. J — Herr Georg Niessing continues his studies 

 on the cells of the testis in the salamander, with special reference to the 

 centrosome, the central spindle, and the structure of the nucleus. In 

 cells in which the nucleus displays a skein-like structure, the centro- 

 sphere is distinct. It contains 2-3 central corpuscles, imbedded in a 

 •clear area, which is separated from the rest of the sphere by a finely 

 granular stratum (membrane ?). In the next stage, where the strands of 

 the nuclear skein loosen, there is little change in the centrosphere, but 

 as the nuclear skein begins to segment, the central corpuscles enlarge, 

 become mulberry-shaped, and finally break up into granules. The 

 granules occupy the polar spindles during division, and ultimately 

 become the microcentres of the daughter-cells. In consequence the 

 author defines central corpuscles as cell-organs in the shape of minute 

 granules, which constitute the point of origin of the fibrillar rays. The 

 connection with the system of fibrillae is to be regarded as an integral 

 part of the definition. The centrosome it is as yet impossible to define 

 strictly, though the term may be used without fear of misunderstanding, 

 the granules just described being known as centrosome-granules. They 

 •unite to form the centrosome proper. As to the nature of the connec- 

 tion between centrosomes and, chromosomes — that is, the origin of the 

 threads of the spindle — the author believes that the spindle is a sup- 

 porting organ consisting partly of an elastic spindle- substance, and 

 partly of contractile threads, fused with the spindle-substance and 

 capable of bringing about changes of form in the spindle. 



In regard to the structure of the nucleus, the author does not believe 



* Morphol. Jahrb., xxvii. (1899) pp. 497-8. 



t Zool. Anzeig., xxii. (1899) pp. 211-6. See Amer. Nat., xxxiii. (1899) pp. 743-4. 



% Arch. f. Mikr. Anat., lv. (1899) pp. 63-110 (1 pi.). 



