290 ZOOLOGY sect 



The black now begins to encroach on the white hemisphere ; 

 cells, budded from the yolk-cells, take on the character of ectoderm, 

 acquire pigment, and gradually extend the black area until it 

 covers the whole embryo except a small patch, known as the yolk- 

 phig (G, H, yk. pL), at what will become the posterior end. This 

 process is obviously one of epiboly : the margin of ectoderm cells 

 surrounding the yolk-plug represents the blastopore. 



The archenteron (I, enQ arises by a split taking place ainong the 

 yolk-cells, beginning at the edges of the blastopore and gradually 

 extending forwards : the process is probably supplemented by a ■ 

 limited amount of invagination of the ectoderm. The archenteron 

 is at first a very narrow cleft, but soon widens considerably: 

 for a long time it does not actually communicate with the exterior, 

 the blastopore being filled up with the yolk-plug. As the archen- 

 teron extends forwards the blastoccele gradually disappears. The 

 yolk-cells soon become differentiated into a layer of endoderm 

 cells (I, end.) immediately surrounding the archenteron, and several 

 layers of mesoderm cells (mes.). Ventrally, however, a large mass 

 of yolk-cells (K, yJc.) remains undifferentiated and serves as nutri- 

 ment to the growing embryo. 



The edges of the lower margin of the blastopore now begin to 

 approach one another, and uniting in the median plane, give rise 

 to a vertical groove, the primitive groove. In the meantime 

 medullary folds (H, md.f.) appear and mark the dorsal surface : they 

 are at first widely separated, but gradually approach one another 

 and close over the medullary groove (md. gr.), thus giving rise to 

 the central nervous systeiQ. Posteriorly they become continuous 

 with the lips of the blastopore, so that when the neural groove 

 becomes closed in behind, the archenteron, as in Amphioxus, 

 communicates with the neurocoele by a neurenteric canal (K, n. e. c). 



The embryo soon begins to elongate; one end is broad, and, 

 becoming separated by a slight constriction, is marked out as the 

 head : the other end is bluntly pointed and is the rudiment of 

 the tail (t.). On the ventral surface of the root of the tail aprocto- 

 dceum (pcdm.) appears and communicates with the archenteron. 



The head and tail become more distinctly marked off from the 

 trunk. A pit — the stomodcBioiii (J — L, st. dm.) — appears on the 

 antero -ventral surface of the head, and, immediately behind it, a 

 semilunar area with raised edges, the sucker (sk). At each side 

 of the head two branched processes appear : they are the external 

 gills (hr'^., br^.), and the regions from which they arise mark the 

 positions of the first and second branchial arches. 



The embryos are now hatched as tadjjoles. They swim freely 

 in the water or adhere to weeds by means of their suckers 

 (Fig. 940, 1). They are still blind and mouthless, the stomodseum 

 not having yet communicated with the archenteron. Soon a third 

 pair of external gills appears on the third branchial arch, and the 



