So 



SPONGES 



not very intelligible. According to Metschnikoff, the blastula becomes 

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

 when hatched is solid, with an inner mass enveloped in a layer of 

 flagellated cells which show a differentiation at the hinder end of the body. 

 According to Barrois the development is similar to Oscarella. Not much 

 can be drawn from the development of either of these important forms at 

 present. 



In the Monaxonida and Keratosa a highly specialised but 

 essentially simple type of larva is found. The segmentation of 

 the ovum is total but unequal, 1 resulting in the formation of a 

 compact mass of centrally placed macromeres, completely or partially 

 surrounded by a superficial layer of micromeres (Fig. 62, A}. The 

 blastomeres next become differentiated in situ to form the larva. 

 The micromeres develop into the flagellated gastral cells. The 



FIG. 62. 



Two stages in the prelarval development of Chalinula fertilis. A, stage in the segmentation ; 

 B, later stage in which the histogenesis of the larva is advancing, mic, micromeres ; mac, macro- 

 meres ; c.c, ciliated cells ; i.m, inner mass ; spic, spicules. (After 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 

 amoebocytes (Fig. 62, B}. In short, both blastogenesis and histo- 

 genesis take place 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 (Dictyo- 

 ceratina, Spongilla ; cf. Fig. 59, 6), or leaving it exposed at the 

 posterior pole (Halichondrina, Cliona ; cf. Fig. 63, A, and Fig. 59, 

 5). 2 The larva is therefore perfectly comparable to a parenchymula 



1 It may be doubted, however, if the unequal size of the blastomeres is really to 

 be explained as due to a process of meroblastic segmentation comparable to that 

 induced by the presence of food-yolk in many Enterozoa. It is more probable that 

 it is simply due to the fact that the cells destined to give rise to the (smaller) gastral 

 cells divide up oftener than those destined to form (larger) dermal cells. 



2 In Aplysilla the inner mass is said to protrude at the anterior pole (Delage). 



