13'i , WILLIAM BATESON. 



occur in the external appearance of the embryo from this 

 point until the form shown in fig. 16 is reached. 



For convenience of reference I shall allude to the Stage 

 shown in fig. 7 as Stage A; iu fig. 9 as Stage B ; in fig. 10 as 

 Stage C; in fig. 11 as Stage I); in fig. 13 as Stage E; in fig. 

 14 as Stage F ; in fig. 15 as Stage G ; in figs. 16 and 17 as 

 Stage H. 



The Grastrula. — The blastosphsere is at first spherical. It 

 next assumes an elliptical form and gradually becomes flattened 

 on one side (figs. 18 and 19). The flattened side next 

 becomes concave and is rapidly invaginated, forming a cup- 

 shaped gastrula (figs. 5, 20, and 21). This gastrula is 

 radially symmetrical and the blastopore is still circular. The 

 edges of the blastopore then approximate, and during this 

 process the embryo grows unequally, causing one of its axes to 

 elongate slightly (figs. 6 and 7). As a result of these 

 changes the blastoporic aperture forms a very short slit, placed 

 in a depression lying rather towards the pole which afterwards 

 forms the posterior end of the animal. As the closing of the 

 blastopore proceeds the whole surface of the body becomes 

 covered with very minute cilia and a ring of larger cilia 

 develops round the blastopore which is placed in a slightly 

 eccentric position within it (figs. 7 and 8). 



Stage A is thus reached. When the cilia appear, the larva 

 rotates in the eggshell on the blastoporic axis in the direction 

 of the hands of a watch. 



Stage B is attained merely by an elongation of the axis at 

 right angles to the plane of the ring of cilia, whose position 

 makes the determination of the approximate position of the 

 blastopore possible throughout larval life (fig. 9). An examina- 

 tion of a series of sections through a larva of this stage shows 

 that the blastopore has closed. 



The changes from Stages B to F occupy about fifteen hours. 

 In Stage C the long axis is still more marked. 



In Stage D (figs. 11 and 12) the body is further elongated 

 and slightly flattened, the flat surfaces being afterwards shown 

 to be dorsal and ventral respectively. A transverse constriction 



