538 



CEPHALOCHORDA. 



cleavage, the circle of small blastomeres divides more rapidly, while 

 the circle of eight larger cleavage-spheres surrounding the vegetative 

 pole remains longer undivided (Fig. 279 G). In later stages, the 

 regular arrangement of the blastomeres in circles is obliterated and 

 the cells form an epithelium surrounding the cleavage-cavity, the 

 Uast'idctrstage being reached in this way (Fig. 279 /, K). The eoo- 

 at this stage, is lengthened in the direction of the future gastrula- 

 axis, and the wall at the vegetative pole, i.e., the posterior third of 

 the egg, is composed of somewhat larger cells richer in yolk-granules. 

 This represents the entodermal region of the embryo. In it flatten- 

 ing occurs, and this soon passes into an invagination (Fig. 280 A) 

 which leads to the development of a cap-shaped gastrula. The 

 invagination causes the cleavage-cavity to decrease in size and finally 

 completely to disappear, the two primary germ-layers coming into 

 close contact (Fig. 280 B). 



C 



Fig. 280. — Three consecutive ontogenetic stages of Amphwxus (after Hatschek), 

 showing the invagination of the entoderm. A, 'luring invagination ; B, after the 

 completion of invagination, right dorsal side, left ventral side ; (J, with narrowed 

 gastrula-mouth, orientation as in /!. 



The gastrula-stage now passes through certain phases by means of 

 which the bilateral symmetry which, according to Wilson, is already 

 evident in the stages of cleavage, becomes more distinct, while at the 

 same time, the embryo elongates in the direction of its definitive 

 longitudinal axis. The apex of the ectoderm of the gastrula corre- 

 sponds to the animal pole, while the vegetative pole may be said to 

 coincide with the centre of the at first circular aperture of invagina- 

 tion. This latter soon becomes oval, and the plane of symmetry is 

 thus established. These later stages, seen in profile (Fig. 280 £), 

 show a point at which the curve is more abrupt ; this point does not 

 coincide with the animal pole but lies somewhat excentrically, corre- 

 sponding to the anterior end of the later principal axis, the posterior 



