TRACE EAT A. 321 



The most important sources of information for the general embry- 

 ology of the Chilognatha are the papers of Newport (No. 397) and 

 Metschnikoff (No. 398). The development of Strongylosoma may 

 be taken as fairly typical for the group ; and the subsequent state- 



c B A 



FIG. 173. THREE STAGES IN THE DEVELOPMENT OF STRONGYLOSOMA GUERINII. 

 (After Metschnikoff.) 



A. Embryo on eleventh day with commencing ventral flexure (a;). 



B. Embryo with three pairs of post-oral appendages. 



C. Embryo with five pairs of post-oral appendages. 



gs. ventral plate; at. antennae; 1 -5 post-oral appendages ; x. point of flexure of 

 the ventral plate. 



pallipes, and though carried on by means of sections, still leave some points very 

 obscure, and do not appear to me deserving of much confidence. The two species of 

 Julus and Craspedosoma undergo, according to Stecker, a nearly identical development. 

 The egg before segmentation is constituted of two substances, a central protoplasmic, 

 and a peripheral deutoplastic. It first divides into two equal segments, and coiu- 

 cidentally with their formation part of the central protoplasm travels to the surface 

 as two clear fluid segments. The ovum is thus composed of two yolk segments to two 

 protoplasmic segments. The two former next divide into four, with the production of 

 two fresh protoplasmic segments. The four protoplasmic segments now constitute the 

 upper or animal pole of the egg, and occupy the position of the future ventral plate. 

 The yolk segments form the lower pole, which is however dorsal in relation to the 

 future animal. The protoplasmic segments increase in number by a regular division, 

 and arrange themselves in three rows, of which the two outermost rapidly grow over 

 the yolk segments. A large segmentation cavity is stated to be present in the interior 

 of the ovum. 



It would appear from Stecker's description that the yolk segments (hypoblast) next 

 become regularly invaginated, so as to enclose a gastric cavity, opening externally by 

 a blastopore; but it is difficult to believe that a typical gastrula, such as that represented 

 by Stecker, really conies into the cycle of development of the Chilognatha. 



The mesoblast is stated to be derived mainly from the epiblast. This layer in the 

 region of the future ventral plate becomes reduced to two rows of cells, and the inner 

 of these by the division of its constituent elements gives rise to the mesoblast. The 

 development of Polydesmus and Strongylosoma is not very different from that of Julus. 

 The protoplasm at the upper pole occupies from the first a superficial position. 

 Segmentation commences at the lower pole, where the food yolk is mainly present ! 

 The gastrula is stated to be similar to that of Julus. The mesoblast is formed in 

 Polydesmus as a layer of cells split off from the epiblast, but in Strongylosoma as an 

 outgrowth from the lips of the blastopore. Stecker, in spite of the statements in his 

 paper as to the origin of the mesoblast from the epiblast, sums up at the end to the 

 effect that both the primary layers have a share in the formation of the mesoblast, which 

 originates by a process of endogenous cell-division ! 



It may be noted that the closure of the blastopore takes place, according to Stecker, 

 on the dorsal side of the embryo. 



B. E. *2l 



