ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 189 



seasons of the year; little is known as to the characters of freshly- 

 deposited ova, but, from the evidence which he has been able to collect, 

 the author is of opinion that ova are deposited throughout the year, and 

 that there is no special breeding season. 



Maturation and Fertilization of Ovum in the Lamprey.* — Herr 

 A. A. Bohm gives a more detailed account than that previously published 

 of his investigations on the maturation and fertilization of the ovum of 

 Petromyzon planeri. 



A. Maturation. — An account of previous observations is as usual 

 prefixed. The author then follows Rathke in a description of the ovary. 

 In young Ammoccetes, 5 cm. in length, the ovary is a single median sac. 

 An apical portion, bent sideways, v^as the seat of formation of new ova ; 

 in the main median portion all the ova were of the same stage. The 

 evolution of the ovarian folds is then traced. In the last months before 

 metamorphosis it was seen that all the ova were arranged in the same 

 way, with the vegetative pole towards the body-cavity, and the animal 

 pole (with the approximated nucleus) towards the axial blood-vessels. 



In Ammoccetes 5 cm. in length the ova exhibited a central germinal 

 vesicle ; they grow continuously to the time of metamorphosis ; the yolk 

 appears while the nucleus is still central. At the time of metamor- 

 phosis, the ova are inclosed in a double membrane, and this by a layer 

 of granulosa. The substance exhibits crystal-like yolk-granules and 

 numerous " vacuoles," perhaps of connective material like that which 

 sparsely unites the granules. A two-layered cortical zone and a pellucid 

 central area are conspicuous. The nucleus becomes eccentric in position. 

 Between it and the surface lies a peculiar disc-like mass, the lid of 

 A. Miiller. This is only a transitory structure, not seen in the ovarian 

 ova of mature lampreys, in which the germinal vesicle is quite superficial 

 and polar. The granulosa undergoes mucous degeneration, more marked 

 at the vegetative pole. In the freed ovum within the body-cavity the 

 karyoplasma of the germinal vesicle expands like a cup, and forms the 

 " pole-plasma." 



B. Fertilization. — The author gives a report of the results reached by 

 A. Miiller, Calberla, Kupffer, and Benecke. He then proceeds to detail 

 his own observations at successive periods, first of minutes, and then of 

 hours after fertilization. The fresh laid egg exhibits a mucous envelope, 

 and at the animal pole a hyaline cupola. This is situated on a watch- 

 glass-like arch of the egg-shell, which here as elsewhere consists of an 

 internal radially porous, and an external homogeneous layer. On such 

 unfertilized eggs no micropyle is to be seen. Several spermatozoa enter 

 at the cupola ; elsewhere the ovum is impenetrable. Within the cupola 

 the spermatozoa are to be seen, which steer towai'ds the centre of the 

 ovum. Only one, however, penetrates. But, before the spermatozoa have 

 touched the egg-membrane, a constriction is formed at the margin of the 

 watch-glass-like elevation, the pole-plasma separates from the membrane, 

 and forms in so doing a space traversed by numerous thin threads. Even 

 this several spermatozoa may reach, but get no further. 



During the further retraction of the ovum, in the region of the pole- 

 plasma, the above-mentioned threads and a thicker axial strand are 

 withdrawn into the main mass, which has meanwhile assumed a spherical 

 form. At the same time the first polar body is extruded. The yolk- 



* Aicb. f. Mikr. Anat.. xxxii. (1888). pp. 613-70 (2 pis.)- 



