ARACHNIDA, 



before it has grown round the latter, however, the rudiment of the 

 germ-band has appeared, and the first differentiation of the latter 

 takes place at the point where the blastoderm first began to form. 

 A cleavage of the yolk, such as is met with in the eggs of the 

 Araneae, does not occur in the Scorpiones. 



The discoidal cleavage of the Scorpiones might be compared with the Crusta- 

 cean method of cleavage distinguished as Type IV., and might, like the latter, 

 be traced back to superficial cleavage (Vol. ii., pp. 117 and 118). This would 

 be the more permissible as superficial cleavage is, as a rule, widespread among 

 the Arachnida also. In this respect the Scorpiones, as compared with the 

 Araneae, must be considered as showing a modified condition, although they 

 are in other respects more primitive. The development of the embryo within 

 the body of the mother is a sufficient proof that modification in the primitive 

 method of development has taken place. 



The Formation of the Germ-layers. The germ-disc does not long 

 retain the character of a single layer of cells. A thickening appears 



at its centre, which, on the sur- 

 face turned towards the yolk, 

 appears as a swelling. This, 

 according to Kowalevsky and 

 Schulgix, has arisen by a down- 

 sinking of the cells. If we bear 

 in mind, in addition to this, the 

 longitudinal furrow described by 

 Metschxikoff on the surface of 

 the now oval germ-disc (Fig. 4 A, 

 p. 6), we are reminded of the 

 long slit -like blastopore that 

 occurs in Peripatus and in the 

 Insecta, and which constitutes 

 the longitudinal germinal groove. 

 In any case, the differentiation of 

 the inner and middle germ-layers 

 starts from this point. 



Fig. 1. — Egg of Euscorpius italicus showing 

 the germ-disc (after Metschnikoff, from 

 Balfour's Text-book). 



Laurie (No. 23), in his recent work, does not actually deny the " down- 

 sinking " of the cells and the presence of the longitudinal furrow, but being 

 unable to convince himself, seems inclined to doubt their existence. This, indeed, 

 cannot be considered as established, especially as the descriptions given of these 

 processes are not very exact. Laurie derives the ento-mesoderm by delamination 

 from the cell-mass of the germ-disc, in which no special regularity of structure 

 is apparent. But he also finds a thickened point at the posterior end of the 

 germ-disc, in which rapid increase of cells takes place (formation of the ento- 

 mesoderm), and which could therefore be compared with an invagination (Fig. 3, 



