of a Species of Chironomus. 43 



vitellus increases, it collects in the lower extremity of the 

 ovum ; and the nucleus, which is already the germinal vesicle, 

 descending from the upper extremity, buries itself in the con- 

 stantly increasing vitelline mass. The chorion, however, is 

 formed by the activity of the epithelial layer of the tunica 

 propria] but whether this structure is formed as a cuticular 

 deposit of the epithelial cells, as described by Leydig*, or the 

 epithelial cells are directly converted into the chorion, as stated 

 by Steinf, I am unable to say J. 



It is therefore clear that the germinal vesicle of the ovum 

 has originated from the nucleus, and the vitellus with the oil- 

 drops and the chorion (corresponding to the epithelial cells of 

 the tunica 'propria) from the vitelligenous cells, which repre- 

 sent the embryonal cells of the ovarian tube. But when we 

 remember that the ovarian tube has been produced by the 

 conversion of the polar cell, that the nucleus of the former 

 {i. e. the ovarian tube) is only a portion of the nucleus of the 

 latter {i. e. the polar cell), and the nucleus of the polar cell, 

 again, is only a part of the germinal vesicle, we become con- 

 vinced that this generation stands in direct connexion with 

 the preceding one, and that its germinal vesicle is only a part 

 of that of the first §. 



W. Keferstein in 'Zeitschr. fur rat. Medicin,' 1862, Bd. xiii. pp. 198, 199). 

 The same opinion is also partially supported by Claus, who says that 

 ^' epithelial cells, vitelligenous cells, and ova " (i. e. the germinal vesicles ; 

 but we have already seen that these are of different origin from the 

 vitelligenous cells, and therefore we cannot agree with him) " are modi- 

 fications of originally homogeneous elements, that they have proceeded 

 genetically from the same cells, and by a different mode of development 

 have attained such divergence of form" (I. c. p. 44). Stein thinks that 

 the epithelial cells also take part in the formation of the vitellus ; and 

 Leydig is of opinion that they only secrete the chorion (Der Eierstock 

 &c. p. 57) ; but nevertheless he admits their affinity to the ova in other 

 animals, resting his opinion upon the investigation of La Vallette {ibid. 

 p. 56, note 1). With regard to Metschnikow's opinion, vide supra. 



* Leydig very accurately describes the development of all the layers 

 of the chorion in Timarclia tenehricosa (see his ' Eierstock und Samen- 

 tasche,' pp. 11, 14, and 57, Taf. 2. figs. 7-10). t Leydig, I. c. p. 59. 



X Meyer ("Ueber die Entwicklung des Fettkorpers, &c.," Zeitschr. fur 

 wiss. Zool. Bd. i. p. 193) says that when the vitellus collects, the epithe- 

 lial cells divide in the direction of the radii of the ovum, and lie with 

 their outer ends on the chorion, and " strengthen it," and afterwards, 

 " whilst the epithelial cells amalgamate with the chorion, they become 

 thick- walled, unite firmly with each other, and lose their nuclei." 



§ M. Ganin (" Beitriige zur Erkenntniss der Entwicklungsgeschichte der 

 Insecten," Zeitschr. fiir wiss. Zool. 1869, p. 387) says : — " At any rate, it is 

 clear that both the central cell and its nucleus (from which the embryo 

 is developed) must be regarded as new formations." It seems to me, 

 however, that this opinion is by no means correct, any more than Weis- 

 mann's theory of the fi-ee formation of the germ-cells, which has recently 

 been supported by Ganin in the Pteromalinse {ibid. p. 439). 



