182 ZOOLOGY SECT. 



Development. Segmentation is meroblastic, 1 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 (Fig. 839) now begins at the thickened embryonic 

 edge of the blastoderm, which here becomes continuous with the 

 cells of the lower layer. The cavity (al), at first very small, 

 formed below this in-folding is the rudiment of the archenteron, 



Fio. 839. Longitudinal section through the blastoderm of a Fristiurus embryo before the 

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

 invagination. al. archenteron ; ep. ectoderm ; er. embryonic rim ; /;?. niesoderm. (After 

 Balfour.) 



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 estab- 

 lished at about the same time. It is formed from two separate 

 and distinct sources (Fig. 840). Along the edge of the embryonic 

 rim appears a horizontal groove-like depression : this the 

 external ccelomie lay (c.l). 1 ) marks the line of origin of the 

 peripheral part of the mesoderm (m.s. 1 ), which grows inwards from 

 it as a plate of cells between the ectoderm and the endoderm. 

 The central part of the embryonal mesoderm (m.s. 2 ) is developed 

 from the endoderm at a point immediately external to the 

 rudiment of the notochord : here also a slight groove the internal 



1 Except in one species of Cestraciou. 



