1877.] 179 [McCrady. 



of protoplasm is largely increased, as in the case of the growth of 

 Amoeba. The blastoderm, on the other hand, also grows, both by 

 the increase of its cells in number (by fission, etc.) and in actual 

 bulk. Part of this increase, and in many cases the far greater part, 

 as in the bird's egg, is no doubt due to the material assimilated from 

 the yolk alone ; but there are numerous examples among the lower 

 marine embryos of growth, which can hardly be ascribed to this 

 source only ; and we must suppose that, like many protozoa, such 

 protembryos also increase by the organic matter suspended or dis- 

 solved in the water; or that their protoplasm possesses also the veg- 

 etable power of building up inorganic matter into protoplasm. When 

 in animals chlorophyl is present, we have a right to assume such a 

 power, and certainly no definite line of demarcation can be drawn 

 between animals and plants, either in generation or development. 



24. According to the provisional theory just explained, Protozoa 

 are organic forms in which the animal is its own sexual product, and 

 they might on this account be called Oozoa, which would be a prefer- 

 able name, as it does not prejudge the question as to the order of 

 appearance of these animals in the sequence of life. 



25. On the other hand, every egg and every spermatozoon may be 

 regarded as an individual protozoon. In sponges Hgeckel has figured 

 the yolk protoplasm of the egg as putting forth pseudopodial lobes 

 and behaving like an Amoeba. In the spermatozoa of Ascarides, it has 

 been long known that they not only resemble an Arcella in general 

 form, but move also in a rhizopodous fashion. The egg of no ani- 

 mal is more motionless than an encysted protozoon; and, indeed, eggs 

 provided with vitelline membrane only, or with this and other envel- 

 opes, may be regarded as encysted protozoa. 



Again, spermatozoa under their ordinary forms may be regarded 

 as flagellate protozoa. 



26. The sexual maturity of a higher organism (Metazoon) then 

 means its arrival at a stage of development when it produces unicel- 

 lular protozoid buds (ova and spermatozoa), each of which is the vehi- 

 cle of a sexual product — germinative vesicle or nucleus of sperma- 

 tozoa. 



27. Isolated, these sexual products are incapable of development; 

 but they have a strong attraction for each other and unite with great 

 energy, compelling as in Protozoa a conjugation of their protozooid 

 vesicles. They may therefore, and indeed must be, regarded as 

 oppositely polarized protoplasms, whose combination results in an 



