of the Ova 0/ Chthonius. 203 



The segmentation takes place quite regularly and very 

 rapidly ; the nutritive vitellus breaks up into two, then into 

 four, and finally into eight segments (segmentation-spheres). 

 The division takes place by the formation of invaginations at 

 the two poles of the &gg, appearing first at the pedicular pole. 

 It is true that I have found an ovum with invaginated poles 

 even in the ovary ; but the first indications of the process of 

 segmentation, in by far the greater number of cases, are only 

 to be met with on the ventral surface of the mother, so that 

 the segmentation in Chthonius is a process occurring in de- 

 posited eggs. With the division of the nutritive vitellus, the 

 brown nucleus, which is pushed a little to one side, also breaks 

 up into two halves, one of which pertains to each segment ; 

 but with this at the same time a division of protoplasm is 

 closely connected, and, indeed, the protoplasm divides into 

 three portions ; one part occupies the cavity between the two 

 segmentation-spheres and remains there, surrounded by a layer 

 of the primary deutoplasm-spheres, throughout the whole pro- 

 cess of segmentation. Of the rest of the protoplasm, nearly 

 equal portions collect round the nuclei of the two vitelline 

 segments. We have then in Chthonius an internal cavity 

 which is at the same time a reservoir of protoplasm, to be 

 afterwards separated from this cavity in order to surround the 

 nutritive vitellus (fig. IV.). 



Now a further division of the two spheres of segmentation 

 into four takes place, being effected by a transverse invagina- 

 tion of the two vitelline cells. Both in this and in the fol- 

 lowing stage, in which the vitellus is divided into eight seg- 

 ments, the nucleus, and consequently also the protoplasm, like- 

 wise divides into four and then into eight parts, so that in the 

 last-mentioned stage eight segmentation-spheres, each with a 

 nucleus which is surrounded by a layer of protoplasm, may be 

 distinguished. The structures detected by Metschnikotf * in 

 the eggs of Ghelifer with four so-called spheres of segmenta- 

 tion, namely the round brown spots consisting of fine granules 

 (representing the cell-nuclei according to Metschnikoff"), are 

 therefore to be regarded as equivalent to the nuclei composed 

 of the coarser protoplasmatic granules. 



When the vitellus has passed through the process of seg- 

 mentation up to this point, a new and very important process 

 commences — namely, the separation [Ausscheidung) of the 

 protoplasm, which is, so to speak, a preparatory process to 

 the formation of the blastoderm. After the vitelline membrane 

 has removed considerably from the large vitelline cells situated 

 in the centre of the &g^, several protoplasm-spheres, which, 



* Metschnikoff, " Entw. des Chelifer,'' I. c. p. 515, pi, xxviii. figa. 4-7. 



