So 



SPONGES 



not very intelligible. According to Metschnikoff, tlie blastula becomes 

 tilled at an early period by "rosette cells" (archaeocytes ?). The larva 

 when hatched is solid, with an inner niai^s enveloped in a layer of 

 flagellated cells which show adifterentiation at the hinder end of the body. 

 According to Barrois the devel^jmient is similar to Oscarella. Nut much 

 can be drawn i'roin the development of either of these important forms at 

 present. 



In the Monaxonida and Kerafosa a highly specialised hut 

 essentially simple type of larva is found. Tlie segmentation of 

 the ovum is total but unequal,^ resulting in the formation of a 

 compact mass of centrally placed macromeres, completely or partially 

 surrounded by a superficial layer of micromeres (Fig. G2, A). The 

 blastomeres next become differentiated in sifit to form the larva. 

 The micromeres develop into the flagellated gastral cells. The 



>^ 







9 'rf' 



^8/; 



>i'- 



■im(mac) 





Kic. 02. 



Two stages in tlie prehirval (levelopnieiit of Chaiiiinla fertilis. A, sta.se in the sejinienlation ; 

 D, later staj;e in wliioh the histojjenesis of the larva is advancing:, mic, micromeres ; iiuic. macro- 

 meres ; r.r, ciliated cells ; i.m, inner mass ; ,«jn'c, spicules. (Atter Maas.) 



macromeres, destined to become the dermal layer, do not re- 

 main uniform in character, but assume the structural peculiarities 

 of tissue cells of the adult, such as scleroblasts, contractile cells, 

 epidermic cells, etc., some finally remaining undifferentiated as 

 ainoobocytes (Fig. G2, B). In short, both blastogeiiesis and histo- 

 genesis take {)lace during the embryonic period. The larva when 

 set free has an enveloping layer of flagellated gastral cells, distin- 

 guished from the other cell-elements by the minuteness of their 

 nuclei, and either completely enveloping the inner mass (Didyo- 

 ccratina, Sjmifjilla ; cf. Fig. 59, 6), or leaving it exposed at the 

 posterior pole {HoHchimtlriiia, L'liona ; cf. Fig. 63, J, and Fig. 59, 

 5).- The larva is therefore perfectly comparable to a parenchymula 



' It may Ik- douhteil, however, if the unefiual .size of tlie lilastonieres is really to 

 ho explainiMl as due to ii i)rocess of inerohliistii' segmentation coniparahle to tliat 

 iiiiluee<l Vty the presence of food-yolk in many Knterozoa. It is more probalde that 

 it is .siniidy <Uie to the fact that the cells destined to give rise to the (smaller) gastral 

 cells divide np oftener than those tlestined to form (larger) dermal cells. 



- In Ajth/silld the inner mass is said to j)rotnide at the anterior pole(Delage). 



