of a 8j>ecies of Chironomus. 109 



the germinal vesicle, pass from the nutritive vitellus into the 

 layer of formative vitellus, at the same time continually 

 dividing. Here each germ-nucleus is surrounded by a layer 

 of the formative vitellus, so that the germ-nuclei are converted 

 into cell-nuclei, and the layer of formative vitellus becomes a 

 cell-layer, which may be described as the blastoderm. The 

 development of the blastoderm commences, however, in the 

 lower pole of the ovum ; that is to say, the germ-nuclei make 

 their appearance in the inferior polar space sooner than in the 

 superior. The cells of the blastoderm, the nuclei of which, as 

 is well known, are strongly refractive, divide in the direction 

 of the radii of the ovum, so that the blastoderm soon appears 

 as a layer of elongated, cylindrical cells. After the comple- 

 tion of the longitudinal division of these cells they divide 

 transversely, so that from the originally one-layered blasto- 

 derm we get a two-layered structure. The cells of the lower 

 blastodermic layer now formed continue dividing in the same 

 direction, so that this layer soon ajDpears as a multistratified 

 cell-mass, the outer layer retaining its original character and 

 its cells not dividing. In consequence of this the boundary 

 between these two blastodermic layers is easily recognized. 



Having now briefly described the formation of the blasto- 

 derm, we venture to raise the question whether this is not 

 identical with the so-called segmentation of the vitellus in 

 other animals ? We know that two kinds of ova are distin- 

 guished among animals, — those which only contain the for- 

 mative vitellus being designated holohlasticj and the others, 

 which have both the formative and the nutritive vitellus, 

 meroblastic ova. The insect-ovum, however, possesses at 

 first only one sort of vitellus, which subsequently divides into 

 the nutritive and the formative vitellus. Hence the insect- 

 ovum may be regarded first as holoblastic, and afterwards as 

 meroblastic. The insect-egg, therefore, miites these two 

 kinds of ova with each other, representing a transition form. 

 We know, further, that the segmentation of the ovum is either 

 total, as in the holoblastic ova, or partial, as in the meroblastic 

 ova. Both consist in the division of the first sijliere of seg- 

 mentation (in which there is a nucleus with a nucleolar cor- 

 puscle, which must probably be regarded as the germinal 

 vesicle*) into a great number of small spheres ; this process is 

 most properly interpreted by Kollikerf as '' a kind of cell- 

 multiplication process." Does not this take place also in 

 insects ? Have we not seen the division of the germinal 



* According to Johannes Miiller, Gegenbaur, and Leydig. 

 t Entwickelungsgeschichte des Menscheu iind der lioheren Tliiere, 

 p. 30. 



