466 



LECTURE XIX. 



Embryo spider. 



first great segment of the body in the mature spiJer. The margins 

 of the thorax are next seen to be subdivided on each side by three 

 parallel fissures into four segments (1, 2, 3, 4.) ; these are the basis 

 of the epimeral pieces. The part of the opaque j-^ , 

 integument which connects the two series below ^^ 



is the rudimental sternum. A second constric- 

 tion begins to divide the thorax from the abdo- 

 men ; the mandibles or antennce {fig. 177, b b) 

 begin to bud forth as two convex processes 

 from the anterior part of the head ; the part 

 intervening between these and the epimeral 

 pieces forms the rudiment of the maxillte (c). 



The intermediate labium also begins to be de- 

 fined from the sternum. The opaque peripheral 

 layer (e), extending from the thorax to the opposite end of the ovum, 

 lays the foundation of the ventral integument of the abdomen. Upon 

 the opaque integument, which is extending backwards over the dorsal 

 part of the head, the characteristic group of simple eyes {d) begins 

 at this time to be distinctly developed, and the rudiments of the 

 maxillary palps and of the four pairs of thoracic legs become recog- 

 nizable, raised in relief upon the epimeral segments ; now, also, the 

 dorsal vessel {f) appears along the upper curvature of the abdomen ; 

 and thus all the chief characteristics of the future spider are mani- 

 fested, whilst the great mass of the vitellus remains still visible 

 through the transparent and incomplete lateral and dorsal parts of 

 the integument. 



The constriction between the two divisions of the body increases ; 

 the legs and palpi next present slight traces of articulations ; as they 

 increase in length they cross the middle line of the sternum and 

 interlock with those of the opposite side. The mouth, the vent, and 

 the wide alimentary canal are formed ; the integument is completed, 

 as in other Articulata, by a dorsal cicatrix, and in this state the 

 young spider breaks tlu'ough the attenuated chorion. The jaw- 

 shaped antennae, the cephalothorax, and abdomen, are first extricated, 

 and afterwai-ds, but with more difficulty, the palpi and legs are with- 

 drawn. A similar process has soon to be repeated in the casting off 

 the foetal integument, which becomes too small for the rapid growth 

 of the young spider. This first moult always takes place in the 

 silken nest of the parent ; the young spider then issues forth, and is 

 subject to repeated moults before acquiring the mature size. We 

 perceive, therefore, that throughout the whole process of the de- 

 velopment of a spider, there is nothing worthy to be called a meta- 

 morphosis. The highest of the Articulata never acquires the condition 



