510 Arthur E. Shipley 



the cells become flatter with a better defined outline, and soon they break 

 from one another and tlien bave an oval form with a large nucleus 

 (%.23). 



The blastosphere becomes slightly flattened at one pole and bere an 

 invaginatiou takes place ;fig'. 24) . The invaginated layer forms three cavi- 

 ties, namely in the centre the meseuteron, andtwo lateral cavities which 

 form the future body cavity. The cavity of the mesenteron is small and 

 that of the body is very small. The blastopore closes, it is nearer the 

 anterior than the posterior end of the embryo, but whether dorsal or ven- 

 tral , I could not determine. The three cavities formed by the invagi- 

 nated layer at first communicate at the end near the blastopore, they sub- 

 sequently become shut off from one another. The centrai cavity forms 

 the alimentary canal : throughout the larvai life it is without a mouth or 

 anus. The walls of the two lateral cavities form the muscles and other 

 mesoblastic structures : the splanchnic and somatic walls are frequently 

 in contact so that the body cavity is obliterated in many places, the 

 latter does not become spacious tili the adult condition isattained. The 

 mesentery is formed by the approximation of the mesoblastic walls, be- 

 fore and behind the digestive tube : in the latter Situation it must be sub- 

 sequently absorbed as it is not present in the adult Argiope, although a 

 dorsal mesentery is found in Waldheimia and Rhynchonella. 



While these internai changes bave been going ou the embryo has 

 divided into two segments. The anterior is connected with the walls of 

 the brood pouch by a fine filament, and it remains so until the free swim- 

 ming larva is ready to leave the mother (fig. 25). 



Soon after the appearance of two segments , the anterior is again 

 transversely constricted, and the larva now consists of three segments, 

 the posterior being somewhat longer and narrower than the other two 

 (figs. 26). About the sanie time four eye-spots appear on the anterior 

 Segment, these are minute aggregations of cells of a deep red color ; the 

 pigment spots are symmetrically arranged. The numberistypically four, 

 but in rare cases six eye-spots are present. Soon after the first appear- 

 ance of the second segment, four bundles of small bristles appear on its 

 edge. 



Fig. 33 represents a longitudinal section through a larva shortly 

 after the appearance of the second segment. It will be seen that this 

 second segment is produced by a fold of the epiblast which extends 

 round the whole larva ; this contains near its edge a small spherical 

 collection of mesoblast cells, which scerete the setae. This fold of epi- 

 blast soon commences to'grow down over the posterior segment, it con- 



