ANIMAL BIOLOGY. 



[Part I. 



ance in cell life, the nucleus. The first thing that happens in 

 development is the division of the original nucleated cell into 

 two nucleated cells, called Uastomeres. 



Once begun the cleavage rapidly continues. The two cells 



become four, the four become 

 eight, the eight become sixteen 

 (Fig. 1, ii. iii. iv.), and so on, 

 until by combined vertical and 

 horizontal cleavage the primitive 

 nucleated cell of the ovum has 

 become split up into a great 

 number of much smaller blasto- 

 meres, each of which is, however, 

 a nucleated cell. The cells in 

 the darker hemisphere are smaller 

 than the cells in the lighter 

 hemisphere; but after awhile the 

 darker portion begins to encroach 

 upon the lighter, and this goes on 

 until the whole surface has become 

 dark, except a little patch, which 

 finally becomes only a small de- 

 pression, called the blastopore (Fig. 1, vi. IL). 



The first indication of the future frog is a broad shallow 

 groove, the edges of which soon rise up to form ridges or folds. 

 It is known as the neural groove, and the ridges as neural folds 

 (Fig. 2, i. ng. and nf.). As development proceeds the folds rise 

 up further, and bending over meet along the middle line, so as 

 to convert the neural groove into a neural canal. The neural 

 tube thus formed will give rise to the brain and si>inal cord of 

 the future frog. 



The body now begins to elongate, becoming at first oval, but 

 soon showing unmistakeable signs of head and tail. The mouth 

 is indicated by a faint depression, behind and on either side of 

 which are two well-marked suckers (5.), tending to run into each 

 other posteriorly in the middle line. In front of the mouth 

 depression (stomodceum) is a fold of skin (ii. fr.), at the upper 



FIG. 1. CLEAVAGE IN FROG'S OVUM. 



i.-iv. Stages with 2, 4 ; 8, and 16 blasto- 

 meres. v. A later stage when the smaller 

 blastomeres cover half the ovum. vi. A 

 still later stage when the smaller blasto- 

 meres have enveloped the whole ovum, 

 except at one spot, the blastopore, bl. 



