268 ZOOLOGY SECT, xin 



over the eggs, and the sperms make their way through the jelly and 

 impregnate them. In a short time the jelly swejlsup and becomes 

 opaque and is thereafter impermeable to the sperms. 



Segmentation begins by a vertical furrow dividing the oosperm 

 into two cells (Fig. 885, A) and soon followed by a second vertical 

 furrow at right angles to the first (B), and then by an equatorial 

 furrow placed nearer the black than the white pole (C). Thus the 

 eight-celled embryo consists of four smaller black cells and four 

 larger white cells. Further divisions take place (D), the black 

 cells dividing rapidly into micromeres (mi.), the white, more slowly, 

 into megameres (mg.) : as in previous cases, the presence of yolk 

 hinders the process of segmentation. The pigmented micromeres 

 (D F, mi.) give rise to the ectoderm, which is many-layered : the 

 megameres (mg.) contribute to all three layers and are commonly 

 called yolk-cells. During the process of segmentation a blastoccelc 

 (E, U. ccel.) or segmentation-cavity appears in the upper hemisphere. 



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- 

 plug (G, H, yk. pi.), 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, ent.) arises by a split taking place among 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, yk) 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 system. 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, 



