PI ROSOMA —EMBRYONIC DEVELOPMENT. 389 



gills, the endostyle and the whole of the alimentary canal degenerat 

 inu' completely, while the muscle-hoops considerably increase in size, 

 and the nervous system develops correspondingly. The "nurse" 

 then, like a swimming bell of a Siphonophoran stock, carries out the 

 locomotory function, while the nutritive and respiratory functions 

 of the whole stock are fulfilled by certain laterally- placed buds (tropho- 

 zooids) on the dorsal outgrowth. 



4. Pyrosoma. 



Ihe development of Pyrosoma from the egg resembles in many 

 respects that of the Thaliacea. Embryonic development takes place, 

 as in them, within the body of the mother and is consequently direct 

 or abbreviated. It even takes place, as at first in the Thaliacea. 

 within the egg-follicle. Pyrosoma is, however, specially distinguished : 

 (1) by the large amount of food-yolk in the egg, which leads to a 

 discoidal cleavage and the development of a germ-disc and (2) by 

 the early asexual multiplication of the embryo. The primary indi- 

 vidual which develops from the embryo and which has been called 

 the Cyathozooid by Huxley, at an early embryonic stage, gives rise 

 by a kind of transverse fission to four more individuals, the first 

 Ascidiozooids of the colony < Fig. 193, etc). 



We owe our knowledge of the embryonic development of Pyrosoma 

 chiefly to Huxlky (No. 72), Kowalevsky (No. 71), and Salenskv 

 X. 71). 



A. Cleavage and Formation of the Germ-Layers. 



Only a single egg matures in the genital rudiment of the A.scidio- 

 zooid which has arisen through budding, as also is the case in the 

 Thaliacea. Part of the remaining cell-material of the so-called genital 

 strand becomes arranged round the egg as the follicle, while another 

 part is used up in forming the rudiment of the testes and of the 

 oviduct which appears as an outgrowth of the follicle. The egg 

 grows greatly by the addition of food-yolk, so that finally the forma- 

 tive yolk and the germ-vesicle within it form a mere prominence 

 upon the large yolk-sphere (Fig. is:; .|). After the oviduct has 

 become connected with the atrial cavity, spermatozoa pass into it 

 and remain in it until the egg is ready for fertilisation, while the 

 oviduct partly degenerates. At the same tiine.au active immigra- 

 tion of follicle-cells takes place into the space extending between 

 tie surface of the egg and the follicular epithelium (Fig. 183 .)./:). 



