A. Hyatt — Larval Theory of the Origin of Tissue. 337 



Dr. Minot (Proc. Bost. Nat. Hist, vol. xix, p. 170) proposed to 

 name the original bisexual nucleus "genoblast," the female 

 part, " arsenoblast," and the male, " thelyblast,'' and these 

 terms have precedence of those we have advanced, or of those 

 proposed by Van Beneden, but we have preferred to use names 

 which retain the word nucleus, as more expressive of the true 

 relations of derivative nuclei. 



If this is true the occurrence of this process of excluding the 

 masculonuclei in the ovum during the agamic stage exhibits 

 an earlier inheritance of a characteristic which in the Protozoa 

 occurs only after and as a result of impregnation, except possi- 

 bly in some of the more specialized Flagellata and Ciliata, 

 where the existence of spermatocysts and spermatozooids leads 

 one to anticipate a corresponding differentiation. The female 

 zoon certainly appears to be in reality an ovum, and to develop 

 like one into a blastula as pointed out by Butschli. 



This view includes some results worthy of attention. The 

 concrescence of Protozoons, as in cases cited by Drysdale and 

 Dallenger, and in some plants where the whole contents of one 

 pair of cells, or more than one pair of cells, are mingled together, 

 is asexual conjugation, but not sexual conjugation. The latter 

 occurs only b} r the exchange of differentiated parts of nuclei, 

 or between the larva-like spermatozoa and the complementary 

 part of the nucleus in the ovum. Thus such forms as Eudorina 

 and Yolvox might be called, on account of their morphology, 

 Blastrea, and could, because of their mode of reproduction and 

 the existence of but one layer in the body wall, be appropriately 

 designated as true Mesozoa. 



With regard to the meaning of the early stages of the ovum, 

 we come nearer to Butschli (Morph. Jahrb., 1884), than any 

 other author, and regard his placula theory as opening a way 

 far more promising than any so far proposed. This author, 

 however, voluntarily rejected the aid of the sponges in his 

 arguments, under the erroneous impression that they were Pro- 

 tozoa and holds an essentially distinct idea of what the placula 

 is. The embryo of the Calcispongian is, according to our 

 opinion, a single layered placula or a Monoplacula, and directly 

 comparable with the undifferentiated fiat colonies of Protozoa 

 which are more primitive than the Blastula form and represent 

 the simplest condition of an autotemnic colony of Protozoa, 

 like Desmarella of Saville Kent, though not possessed of cilia 

 at this stage and therefore more nearly perhaps representing a 

 mass of amoeboid forms. 



The formation of the apical or esoteric cells of the upper 

 layer from the cells of the Monoplacula transforms this stage 

 into a diplopiacula, the older or basal cells becoming our exo- 

 teric cells. True ectoblastic and endoblastic cells first appeared 



Am. Jour. Sci— Third Series, Vol. XXXI, No. 185.— Mat, 1886. 

 22 



