Larval Theory of the Origin of Tissue. 195 



to the ovum after impregnation, and we therefore propose to 

 designate the cell an autotemnon *, in contrast with the 

 embryo, which is more specialized. The least specialized 

 tissue-cells of the mesenchyme differ less from the individual- 

 ized agamic zoons of the Protozoa, while the spermatocysts, 

 as more highly specialized encysted male zoons, retain the 

 cycle of agamic transformations derived from their male Pro- 

 tozoonal prototypes, and are intermediate to the encysted 

 female zoon or ovum. 



The spermatocyst, in other words, is not dependent upon 

 impregnation for its development, and has necessarily retained 

 more of the characteristic successive transformations of the 

 primitive agamic forms than the ovum. This last has become 

 dependent upon impregnation. The tendency to earlier and 

 earlier impregnation in successive generations, and the corre- 

 lative concentration of autotemnic stages, as shown by the 

 fission of the nucleus and exclusion of the polar globules, has 

 finally established the ovum as a more highly specialized 

 form of cell. 



The conditions of fission in the cyst of a Protozoon and in 

 the ovum and spermatocyst are similar as long as the zoons 

 or cells are all similarly confined ; but when they burst the 

 envelope and become free the surrounding conditions differ, 

 and they correspondingly diverge. 



The early encystment of the ovum, the non-production of 

 the colonial form by incomplete fission, the dependence of the 

 feminonucleus upon impregnation, and the great rapidity and 

 extensive character of the changes by which the diploblastic 

 parenchymula and triploblastic gastrula are built up, all show 

 the excessive concentration of development which has taken 

 place, when any blastula is compared with the corresponding 

 forms among the Volvocinge. There is also ' a distinction 

 between the mode of development of the Volvocina and the 

 lower Protozoa, which has, we think, great significance. 

 They have prolonged gestation, and this can be compared 

 with the similar prolongation of the corresponding period in 

 the early inception of the ovum in the Metazoa. 



They are, however, necessarily only single cells. The 

 whole process of segmentation occurs under conditions which 

 effectually protect the earlier stages in the higher Protozoa 

 and in all the Metazoa 5 but, as might have been anticipated, 

 the more specialized IMetazoon elaborates at once and within 

 limits of this early egg stage a fully-formed colony, the blas- 

 tula, whereas the highest and most specialized of Protozoa 



* From avTos, self, and re/xi/co, to divide. 



