of a Species of Chironomus. 37 



chidae, &c. are subject. The ova of the perfectly developed 

 Chironomus are also developed, as we have seen, without 

 fecundation, when they have been removed from the parent 

 organism. 



II. The Development of the Ovary and Ova. 



For the sake of clearness in discussing the developmental 

 history of the ova, I must anticipate a little, and commence 

 my description with the development of the ovary itself. 



We shall see hereafter that the development of the embryo 

 from the unfecundated ovum deposited by the pupa of our 

 Chironomus is perfectly identical with that of the fecundated 

 ova of the imago, which has also been found to be the case 

 with the Cecidomyidse*. We shall see the development of 

 the germ- or blastodermic cells ; we shall see that, of the 

 germ-cell formation, one germ-ball precedes another, inasmuch 

 as it enters earlier into the blastema-layer, and here, sur- 

 rounded by the protoplasm, becomes converted into the nucleus 

 of a membraneless cell ; this cell passes into the inferior polar 

 space of the ovum, and divides here into two and then into 

 four cells, which are indicated as polar cells (fig. 5). Leaving 

 the discussion of the embryonal development for the present, 

 I will now direct attention to these polar cells, as they are the 

 primordial forms of the subsequent generation, the two next 

 generations, the germinal vesicles of which combine, or, in 

 one word, represent the germs of the ovaries and ova f . 



With the advancing division of the germ-cells the bulk of 

 the contents of the ovum increases, so that the polar spaces 

 soon entirely disappear, and the polar cells, which were placed 

 in the inferior, acute polar space, bury themselves in the layer 

 of the formative vitellus or blastoderm. When we trace their 

 destiny further, we find them (at the moment of the produc- 

 tion of the primitive caudal furrow, which soon disappears, 

 and is apparently of no importance in the further development 

 of the embryo, but, according to Weismann, " must only be 

 regarded as the earliest expression of the bilateral type in 



* Leuckart, " Die ungeschlechtliche Fortpflanzung der Cecidomyien- 

 larven," Archiv fur Naturg. 1865, p. 299. 



t To these polar cells, which, according to Weismann (/. c. p. 208), are 

 "so enigmatical," no embryologist, except Prof. Metschnikow, has paid 

 any attention, or, at any rate, only Robin, who has founded upon them 

 his theory of the origin of germ-cells by sprouting. Metschnikow was 

 the first who recognized the polar cells in Simulia and Ceeidomyia as the 

 germs of the sexual glands. (See his ' Embryologische Studien an In- 

 secten," pp. 31-33 & 103-105 ; and Zhurn. M. H. Pr. 1865, Th. cxxvi. 6. 

 p. 113.) 



