1'YUOSOMA EMBRYONIC DEVELOPMENT. 389 



u'ills. the endostyle and the whole of the alimentary canal degenerat- 

 ing 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- 

 :;/y/>/V/x) on the dorsal outgrowth. 



4. Pyrosoma. 



The development of Pyr<>x<tnia 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 

 di.M-oidal 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 

 Asrf(Jto.io(}i(.k of the colony (Fig. 193, etc). 



\Ve owe our knowledge of the embryonic development of Pyrosama 

 chietiv to HUXLEY (No. 72), KowALEVSKY (No. 71), and SALENSKY 



-N.74). 







A. Cleavage and Formation of the Germ-Layers. 



( July a single egg matures in the genital rudiment of the Ascidio- 

 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 

 iiT<>ws oreatly by the addition of food-yolk, so that finally the forma- 

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

 upon the large yolk-sphere (Fig. 183 A). After the oviduct has 

 become connected with the atrial cavity, spermatozoa pass into it 

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

 oviduct pa'rtly degenerates. At the same time, an active immigra- 

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

 the Mirface nf the euu and the follicular epithelium (Fig. 183 A, fz). 



