292 PR. J. B. GATENBY ON THE GERM-CELLS AND 



mitochondrial spheres, so that the hemisphere away from the gastral layer is 

 partly bereft of mitochondria (PI. 20. figs. 10 and 13). 



15. The path of entry of the sperm is marked by an area formed of a 

 chromophobe vacuolated substance, probably intra vitam of a liquid nature. 



16. The oocyte nucleus breaks up and forms two polar bodies during these 

 changes in the spermatozoon (PL 20. fig. 13, PB), and finally the male and 

 female pronuclei grow to the same large size (PI. 21. fig. 18). 



Cleavage and Development. 



1. During cleavage and early development the yolk and mitochondrial 

 granules are sub-equally sorted out among the blastomeres (PI. 21. figs. 19, 

 20, 22, and 21). 



2. During the histogenesis of the two main tissues of the sponge-larva 

 (PI. 23. fig. 28, PC & GO), these mitochondria and yolk-grains (and probably 

 the Golgi elements), though sorted out previously into subequal quantities 

 between the blastomeres, later become more or less numerous, or specially 

 modified or grouped in special regions, as the tissues develop and differentiate. 



3. Such differentiation is not, so far as the granules themselves are con- 

 cerned, in any way traceable to a special segregation during cleavage. 



4. The fixing and staining reactions of the )'olk-spheres of the egg are 

 different from those of the amphiblastula larva, for during differentiation or 

 histogenesis the yolk-spheres become denser and less delicate, especially in 

 the flagellated cells. The same applies, though less well, in the case of the 

 mitochondria. 



The Amphiblastula Larva. 



1. The larva is formed of three sorts of cells : anterior flagellated histo- 

 cytes, posterior " granular " arch eocytes, and inner amoebocytes. All these 

 originate and differentiate from a unilaminar blastula formed of similar cells 

 (PI. 21. fig. 21). For larvEe see PL 23. figs. 28 and 32. 



2. The larva in later stages of its formation becomes partly or wholly 

 surrounded by a. layer of squamous mesogleal (maternal) cells, which are 

 marked by their containing peculiar irregular mitochondria. This nutrient 

 capsule is best developed near the posterior " granular-cell " pole of the 

 larva, and often interrupted on the fhigellated hemisphere (PL 23. figs. 28 

 and 32). 



3. In many cases these maternal cells penetrate and lie among the posterior 

 "granular cells" (PL 23. fig. 28, LGC) ; when the larva breaks away from 

 the sponge the nutrient capsule is left behind. 



4. The histology of the three cell-elements of the larva is as follows : — 

 The flagellated hemisphere is formed of flagellated cells, marked especially 



in the live animal by the possession at their posterior or inner pole of a dense 

 oroup of yolk-spheres which are pigmented (brownish). There is mixed up 



