6 



ARACHNIDA. 



envelopes. Laurie discovered no mesoderm-cells between the embryonic integu- 

 ments. It will only be possible to form a reliable opinion on this subject after 

 the appearance of Kowalevsky and Schulgin's complete work ; as yet we- 

 liave only a preliminary sketch unaccompanied by figures (No. 19).* 



During the formation of the germ-layers and the development of 



the embryonic envelopes the germ-disc changes its shape, becoming 



broader at its anterior end 

 C 

 B /— Y 



(Fig. 4 A). The narrow 

 posterior part becomes 

 greatly thickened through 

 active proliferation of 

 cells. In the middle 

 line there appears on the 

 surface of the disc the 

 groove - like depression 

 already mentioned, which 

 soon disappears again (Fig. 

 4 A, after Metschnikoff). 

 The cephalic region be- 

 comes marked off from 

 the primary trunk-region 

 by a transverse furrow 

 near the anterior end of 

 the germ-disc, and about 

 the same time, or very 

 soon after, a few trans- 

 verse furrows appear fur- 

 ther back, these being the rudiments of the first body-segments 

 and of a large posterior region. This is followed by the separation 

 of further segments from this latter region (Laurie). 



Metschnikoff describes a stage in which the embryo seems divided up into- 

 three primary regions. The anterior region corresponds to the primary cephalon, 

 and the posterior to the post-abdomen ; the middle section is said to give rise to- 

 the remainder of the body. This view could not be definitely proved by 

 Metschnikoff, and it is more probable that all the trunk-segments are derived 

 from the posterior region. These early stages of the segmented germ-band show 

 a certain similarity with the ontogenetic stages of the Trilobita, and might 

 thus give rise to a comparison with these forms (Vol. ii., p. 340). 



In Euscorpius, a stage was observed at which were present the 

 primary cephalic region, a smaller segment following this (that of the 



* [Brauer's (App. to Lit. on Scorpiones, No. II.) observations on the origin 

 of the embryonic envelopes in Euscorpius are in agreement with those of 

 Laurie. Brauer's account of the whole ontogeny is most complete. — Ed.] 



Fig. 4. — Euscorpius italicus. A-C, germ-discs with the 

 median longitudinal furrow (A) and germ-bands (C 

 and C) (after Metschnikoff, from Balfour's Tcxt- 

 hool). In /; and C the neural groove can be seen, and 

 in C the crescent-shaped cephalic pits, and behind 

 them the rudiments of the cepha^-thoracic and 

 abdominal limbs. 



