182 ZOOLOGY srct. 



Development. — Segmentation is meroblastic/ being confined 

 to the germinal (Use, which, before dividing, exhibits amcEboid 

 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 {cd), at first very small, 

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



Fio. 839. — Longitudinal section through the blastoderm of a Fristiums emhryo before the 

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

 invagination, al. archenteron ; e^. ectoderm ; er. embryonic rim ; in. mesoderm. (After 

 Balfour.) 



and the cells lining this cavity above, which fornj 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 evibryonic 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 coelomic hay {c.h.^) — marks the line of -origin of the 

 peripheral part of the mesoderm (m.s.^), 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.^) is developed 

 from the endoderm at a point immediately external to the 

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



' Except intone species of Cestracion. 



