388 CHILOGNATHA. 



B 



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



(After Metschnikoff.) 



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



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



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



gs. ventral plate; at. antenna;; r 5 post-oral appendages; x. point of flexure of 

 the ventral plate. 



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 pro- 

 duction 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 comes into the cycle of development of the Chilo- 

 gnatha. 



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. 



