168 /OOLOGY SECT. 



to that of the foetus. In Mustelus antarcticus the uterus is divided 

 by septa into several compartments, each containing a single 

 foetus. In some of the viviparous forms a distinct, though very 

 delicate, shell, sometimes having rudiments of the filaments, is 

 formed, and is thrown off in the uterus. In the genera Rhinobatus 

 and Trygontfrhina, which are both viviparous, each shell encloses, 

 not one egg, but three or four. Lsemargus appears to differ from 

 all the rest of the Elasmobranchii in having the ova fertilised after 

 they have been deposited as well as in the small size of the ova. 



Development. Segmentation is meroblastic, being confined 

 to the germinal disc, which, before dividing, exhibits amoeboid 

 movements. While segmentation is going on in the germinal disc 

 there appear a number of nuclei, the source of which is not certain, 

 in the substance of the yolk. When segmentation is complete the 

 blastoderm appears as a lens-shaped disc, thicker at one end 

 the embryonic end. It is found to consist of two layers of cells, 

 an upper layer in a single stratum, and a lower layer several 

 cells deep. A segmentation-cavity appears early among the cells 

 of the lower layer; the lower-layer cells afterwards disappear 

 from the floor of this, the cavity then coming to rest directly 

 on the yolk. 



An in-folding of the blastoderm (Fig. 792) now begins at the 

 thickened embryonic edge of the blastoderm, which here becomes 



FIG. 792. Longitudinal section through the blastoderm of a Pristiurus embryo before the 

 medullary groove has become formed, showing the beginning of the process of infolding or 

 invaginatioii. /. archenteron ; <-.p. ectoderm; er. embryonic rim; HI. rnesoderrn. (From 

 Balfour.) 



continuous with the cells of the lower layer. The cavity (al), at 

 first very small, formed below r this in-folding is the rudiment of 

 the archenteron, and the cells lining this cavity above, which form 

 a definite layer, partly derived from the in-folded ectoderm, partly 

 from the cells of the lower layer, are the beginning of the definite 

 endoderm. The edge of the in-folding, entitled the embryonic rim, 

 is obviously the equivalent of .the dorsal lip of the blastopore in 

 Amphioxus. The endoderm and its underlying cavity soon grow 

 forwards towards the segmentation cavity. Under the latter 

 appears a floor of lower-layer cells, but the cavity soon becomes 

 obliterated as the archenteron develops. 



After the formation of the embryonic rim a shield-like embryonic 

 area is distinguishable in front of it, with two folds bounding a 

 groove the medullary groove. The mesoderm becomes esta- 

 blished at about the same time. It is formed from the lower- 



