DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 45 



ingrowth of cells on the ventral side of the blastoderm, and the 

 entoderm, when first appearing, occupies this position in both these 

 divisions of the Arachnida. In the Scorpiones, it forms a regular 

 epithelium, so that it cannot fail to be recognised as a separate 

 germ-layer, but here also isolated cells enter the yolk. All these 

 facts confirm us in regarding the view of the formation of the 

 germdayers adopted by Morin and Kishinouye (Fig. 21 F) as 

 correct. It cannot, however, be denied that the figures given by 

 Schimkewitsch, and especially those given by Balfour, show yolk- 

 cells in earlier stages and further removed from the thickened part 

 of the blastoderm, which might rather be assumed to have remained 

 behind in the yolk at the time of cleavage, than to have become 

 detached from the thickened part of the blastoderm. If this should 

 be the case, the view here taken is not thereby contradicted ; we 

 have then to do merely with single cells which were not utilised 

 in the formation of the blastoderm, and remained behind in the 

 yolk. These cells, as uitellopliags, perhaps render the absorption 

 of the yolk possible. In that case we must assume that they do 

 not later enter into the formation of the entoderm, but probably 

 disintegrate during the gradual disappearance of the yolk, as is the 

 case with corresponding (yolk) cells in the Insecta. 



2. Development of the External Form of the Body. 



The development of the external form of the body has been 

 repeatedly investigated more or less thoroughly in the cases of 

 Agalena, Clubiona, Epeira, Theridium, Lycosa, and Pholcus, and 

 has been found to follow a very uniform course. In spite of this 

 fact, and although a large number of zoologists, among whom we may 

 mention Herold, Claparede, Salensky, Balfour, Schimkewitsch, 

 Locy, and Kishinouye, have investigated the subject, some points, 

 especially in the earlier ontogenetic stages, still remain obscure. 

 The chief difficulty is connected with the early rudiment of the 

 embryo and the first appearance of segmentation. 



At a time when the blastoderm is either approaching completion 

 or is fully developed, there appears (probably on the later ventral 

 side) the prominence known to us as the primitive cumulus, the sig- 

 nificance of which has already been discussed (p. 41, etc.) From this 

 there extends forwards a band which is distinguished by its white 

 colour from the rest of the egg, and is caused by a marked thickening 

 of the blastoderm (Fig. 23 A, Claparede, Balfour). Herold 

 mentions a cometdike structure which arises at an early stage on 



