ii ARACHNIDA. 



form a thickening which, together with the later proliferation of cells at this 

 spot, yields the rudiment of the germ-band. Morin's account also, as far as 

 we can follow it, seems to confirm this, and the figures adopted from him (Fig. 

 21 D-F) show that an accumulation of blastoderm-cells at first lies on the 

 dorsal side of the egg, while at a later stage only a few cells are perceptible in 

 this region. According to Schimkewitsch, the dorsal side of the egg becomes 

 completely denuded of blastoderm, which only later grows out towards it again. 

 We were at first disposed to attribute the absence of blastoderm on the- dorsal 

 side rather to a belated advance of the nuclei out of the yolk, especially as 

 authors state that the formation of the blastoderm progresses from the ventral 

 to the dorsal side. There seemed here to be a distant resemblance to the 

 cleavage and the formation of the blastoderm as observed in the eggs of Scorpio. 

 Further investigation is needed to show whether this conjecture is correct, or 

 whether such a marked redistribution of the blastoderm-cells as is shown in 

 the figures actually takes place. A similar crowding together of the blasto- 

 derm-cells, though not nearly to such a great extent, has also been observed in 

 other Arthropoda (Astacus, cf. Yol. ii., p. 128). 



According to Schimkewitsch, who on this point is essentially in accord 

 with Balfour, the yolk-cells take part to no inconsiderable extent in the 

 formation of the mesoderm, although the chief mass of them is to be described 

 as entodermic. Schimkewitsch, like Balfour, assumes a two-fold origin for 

 the mesoderm, inasmuch as it is formed from the thickening of the ventrally 

 situated blastoderm, especially from the primitive cumulus, and also by the 

 addition of yolk-cells to this thickened region. Certain modifications here 

 appear in individual forms (Tcycnaria, Epeira, Lycosa) ; upon these, however, 

 we shall not enter, as we are unable to agree with this view. Of the two 

 opposed views, the one assuming the existence of yolk-cells giving origin to 

 the entoderm and the mesoderm to some extent, the other deriving both the 

 entoderm and the mesoderm from the blastoderm by a process comparable to 

 gastrulation, the latter appears to us to be by far the more justifiable. This 

 view is confirmed by Kishinouye's recent work (No. 62). This observer found 

 no nuclei in the yolk after the formation of the blastoderm, but observed cells 

 migrating into the yolk from the blastodermic thickening (Figs. 21 and 22). 

 These cells, which become distributed through the yolk, form the entoderm. 

 Further thickening of the ventral region of the blastoderm gives rise to the 

 mesoderm, as was described above (p. 41). The ventral blastodermic thickening 

 known to us as the primitive cumulus is in any case of significance in connection 

 with the formation of these two germ-layers, for it, like the ventral plate (to 

 be described later), appears before the differentiation of the germ -layers 

 (Kishinouye), and not after it, as MoRIN assumed (p. 41). 



When wc trace back the formation of the germ -layers to the 

 blastoderm, we thereby imply that the yolk-cells also arise from 

 the blastoderm. These latter, according to the unanimous opinion 

 of authors, contain, in the Araneae, the rudiments of the whole 

 entoderm, giving rise later to the epithelium of the enteron. If 

 these cells were to remain in the yolk when cleavage takes place, 

 the process of blastoderm-formation would have to be regarded as 

 epibolic, but this is contradicted by what occurs in related forms. 

 The germ-layers are moreover formed in the Scorpiones also by the 



