334 A. Hyatt — Larval Theory of the Origin of Tissue. 



the mesenchyme differ less from the individualized 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 Protozoonal proto- 

 types, and are intermediate to the encysted female zoon or 

 ovum. 



The spermatocyst, in other words, is not dependent upon im- 

 pregnation 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 correla- 

 tive 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 en- 

 velope 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 Volvocinae. There is also a distinction be- 

 tween the mode of development of the VolvocinaB, 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 effectu- 

 ally protect the earlier stages in the higher Protozoa, and in all 

 the Metazoa, but, as might have been anticipated, the more 

 specialized Metazoon elaborates at once and within limits of 

 this early egg stage a fully formed colony, the blastula, 

 whereas the highest and most specialized of Protozoa get no 

 farther than the production of single ova and spermatocysts* 

 or the earliest stages of segmentation during the same period. 



* For results of protection in producing concentration of development see 

 Genesis of Planorbis at Steinheim, Mem. Bost. Soc. Nat. Hist., I Anniv., 1830- 

 1880 ; Fossil Ceph., Mus. Comp. Zool., Proc. Amer. Assoc. Adv. Sci., vol. xxxii, 

 p. 32, also Balfour's Comp. Embryo!. 



