2X2 DR. J. B. GATENBY ON THE GERM-CELLS AND 



embryo that any differentiation of grannies appears, and these differences 

 seem due to slow growth and not to any segregation during cleavage. It 

 is apparent that pari passu with the cleavage of the egg into smaller and 

 smaller blastomeres, the mitochondria either shrink considerably in size or 

 break up into smaller fragments (compare the size of the granules in PI. 21. 

 fig. 19 and fig. 22). It is not true that the mitochondria (chromidia of 

 Jorgensen) disappear in the late blastula-stage, as was thought to be 

 the case (8 a). 



26. Differentiation of the Nuclei of the Blastula-wall heralding the 

 Formation of Tivo Definite Tissues. 



On PI. 21. fig. 21 I have drawn a Grantia blastula at a stage when all 

 the cells appear alike ; the nuclei and the inclusions of the cytoplasm of 

 these cells seemed to me to be similar throughout the entire blastula. On 

 PI. 22. fig. 27 is a blastula fixed at the period when the nuclei of the future 

 flagellated hemisphere have begun to be easily distinguishable: the earliest 

 signs of differentiation of the future flagellated cells is constituted by a 

 tendency for the ceils of one hemisphere to divide more rapidly, and to become 

 more regularly arranged : the peculiar denseness of the nuclei of this layer 

 is a subsequent and rapid differentiation. In PI. 22. fig. 21 the granules in 

 the cytoplasm are not different in either region, and it seems certain that 

 nuclear differentiation precedes or instigates special cytoplasmic activity. 



Soon after the stage depicted in PI. 22. fig. 27, the character of the yolk- 

 spheres becomes altered ; this is indicated by the fact that the yolk-spheres 

 of the embryonic tissues are more persistent and less easily destroyed than 

 those of the egg or earlier blastula. 



Owing to lack of suitable material, there were several interesting problems 

 which I did not examine, but it is hoped to investigate these at some later 

 period. 



27. Discussion. 

 (a) Organ-forming Substances ? 



In this study the behaviour of yolk and mitochondria has been followed 

 out in the development of the sponge Grantia compressa. The introduction 

 of specific and selective staining and fixing methods has enabled me to 

 identify mitochondria, yolk, and G-olgi apparatus. The sponge falls into 

 the group of animal forms v\ hich have been shown to possess the two 

 protoplasmic inclusions. 



The main result of this research, in so far as it touches on the mitochondria, 

 has been to show that they pass through development without losing their 

 individuality, and that they are distributed evenly during cleavage to the 

 blastomeres. No sort of segregation of materials in special blastomeres could 

 be detected with the methods used by me, and I have come to the conclusion 

 that such differentiation of cell-layers and materials as one finds in the 



