312 



AKTICULATA. 



(fig- r 35 A) becomes divided into three lobes, a median (me) 

 and two lateral (pv). These lobes next become completely 

 separated, and the middle one forms the mesenteron, while 

 the two lateral ones give rise to the body cavity, their outer 

 walls forming the somatic mesoblast, and their inner the 

 splanchnic (fig. 135 B). The embryo now elongates, and 

 becomes divided into three successive segments (fig. 135 B), 

 which are usually, though on insufficient grounds (vide Thecid- 

 ium), regarded as equivalent to the segments of the Cha^topoda. 

 The alimentary tract is not continued into the hindermost of 

 them. 



In Thecidium the ova are very large, and development takes 

 place in a special incubatory pouch in the ventral valve. The 

 embryos are attached by suspenders to 

 the two cirri of the arms which immedi- 

 ately adjoin the mouth. There is a nearly 

 regular segmentation, and a very small 

 segmentation cavity is developed. There 

 is no invagination ; but cells are budded 

 off from the walls of the blastosphere, 

 which soon form a solid central mass, 

 enclosed by an external layer the epi- 

 blast. In this central mass three cavities 

 are developed, which constitute the me- 

 senteron and the two halves of the body 

 cavity. Around these cavities distinct 

 walls become differentiated. The body 

 (Lacaze Duthiers, No. 327) soon after 

 becomes divided into two segments, of 

 which the posterior is the smaller. The 

 hinder part of the large anterior segment 

 next becomes constricted off as a fresh 

 segment, and subsequently the remaining 

 part becomes divided into two, of which the anterior is the 

 smallest. The embryo thus becomes divided into four segments, 

 of which the two foremost appear (?) together to correspond to 

 the cephalic segment of Argiope ; but these segments are formed 

 not, as in Chaetopoda and other truly segmented forms, by the 

 addition of fresh segments between the last-formed segment and 



FIG. 135. Two STACKS 



IN THE DEVELOPMENT OF 



ARGIOPE. (After Kowa- 

 levsky.) 



A. Late gastrula stage. 



B. Stage after the larva 

 has become divided into 

 three segments. 



l>l. blastopore ; me. me- 

 senteron ; pv. body cavity ; 

 b. temporary bristles. 



