224 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



by an equatorial furrow nearer the upper than the lower pole. 

 Divisions of both kinds now proceed, the final result being a 

 blastula (blastosphere) (Fig. 66, B), with a small Uastoccele (seg- 

 mentation cavity) near the upper pole. This is roofed by small 

 cells, while its floor and most of the blastula are made up of large 

 yolk-cells, containing food material. 



Invagination (C, D, E) is partly effected by an inpushing of 

 the small cells at one point, and partly by their growing over the 

 yolk-cells elsewhere, leaving, however, for a long time an un- 

 covered circular area, the blastopore. Meanwhile the blastocoele 

 is gradually pushed aside by the archenteron, which occupies the 

 upper pole of the ovum, and is formed partly by the inpushing 

 of the small cells, as above stated, and partly by an absorption of 

 the yolk-cells to form a cavity, and their differentiation to give 

 it definite walls. The small cells, now almost covering the embryo, 

 may at this juncture be termed ectoderm (epiblast), the contained 

 yolk-cells endoderm (hypoblast). The mesoderm (mesoblast) is 

 formed as a layer several cells thick, commencing at the edges of 

 the blastopore and gradually extending over the rest of the 

 ovum below the ectoderm. It is apparently derived from the 

 endoderm. The notochord is formed at the same time, as a 

 median dorsal thickening of the roof of the archenteron. 



2. General Growth. The ectoderm on the upper side of the 

 embryo thickens into a neural (medullary) plate, broad in front, 

 and narrowing behind to the blastopore. The axis of this plate 

 corresponds to the longitudinal axis of the future tadpole, the 

 blastopore being posterior. A delicate neural (medullary) groove 

 appears on the medullary plate. Its side-walls, the neural (med- 

 ullary) folds, grow up and unite together to form a neural tube, 

 the rudiment of the brain and spinal cord. The dorsal side of 

 the embryo is thus indicated, and its head and tail soon become 

 evident, the former being marked by the dilatation of the neural 

 tube into the three brain vesicles in front. The ventral surface 

 bulges out, owing to the food material contained in the endoderm, 

 but it gradually becomes less and less conspicuous. The blastopore 

 closes up, and the cavities of neural tube and gut are then con- 

 nected by a short neurenteric canal. As growth proceeds, the 

 embryo acquires a strong curve, the dorsal side being concave. 

 The sense organs become established, as also do an oval suctorial 

 mouth with horny beaks, and a cloacal aperture. On each side 



