68 



INVERTEBEATA 



CHAP. 



Fig. 45. — Early stages in the development of Aurdia 

 aurita. (After Hein.) 



A, Mastula stage. B, gastmla stage, bp, blastopore ; in, cells 

 budded from the blastula wall whicli migrate into the interior 

 and disintegrate. 



of the stomach, and are discharged into its cavity, — where they are 

 fertilized by spermatozoa of other individuals taken in with sea- 

 water. The fertihzed eggs escape from the mouth but are retained 

 for a considerable period in pockets of the inner surface of the oral arms. 



These pockets can be 

 recognized in the sur- 

 face view as opaque 

 spots. If they are 

 slit open by needles 

 under sea water the 

 embryos can be ex- 

 tracted. 



The embryos can 

 be preserved in cor- 

 rosive subhmate and 

 acetic acid, or in 

 osmium acid, and 

 mounted whole ; or 

 else embedded in 

 celloidin, orientated, 

 and cut into sections 

 parallel to their longitudinal axis. When such sections are examined 

 it is found that the egg segments with great regularity, and that 

 a spherical hollow blastula is formed. The cells forming the outer 

 wall of this blastula bud off other cells which migrate into the 

 interior, and it looks 



as if we were about A ^^^^J>p B ^p_ 

 to witness the forma- 

 tion of a solid planula ; 

 but the cells which 

 thus migrate inwards 

 break up into granules 

 and are absorbed, thus 

 serving as food for the 

 rest. Then, at one 

 end, the cells forming 

 the wall of the blastula 

 are invaginated, and 

 in this way the single- 

 layered blastula is con- 

 verted into a hollow, 

 double -layered structure termed a gastrula. The opening of the 

 invagination is termed the blastopore. The conversion of a blastula 

 into a gastrula is called the process of gastrulation. 



The blastopore never closes and eventually forms the mouth, 

 although it becomes contracted to the finest capillary dimensions. 

 The yolk granules in the cells become absorbed, and the spherical 

 gastrula becomes converted into an oval one, with a broad basal end 



jiem 



nem 



Fig. 46. — The fixation of the free-swimming larva of - 

 Aurelia aurita. (After Hein.) 



A, free-swimming planula. B, stage just after fixation. !)j), blasto- 

 pore reduced to a mere slit, nsm, neinatocysts. 



