Jax. 1902.] BOTANICAL SOCIETY OF EDIXBUKGH 155 



This is all Xature demands ; and this she accomplishes 

 bj the aid of unicellular organisms. All the observed 

 phenomena of development, all those of heredity, are 

 possible in this way.^ Notwithstanding apparent com- 

 plexity, the process is simplicity itself, the simplest kind 

 of continuity conceivable. 



On the circle of life revolves the epicycle of the germ- 

 cells. The circumference of the former is filled in by an 

 uninterrupted succession of such epicycles. The constant 

 sequence of these is the rhythm of reproduction, the gamut 

 of life. 



EXPLANATION OF FIGURES. 

 Fig. 1. Diagram of the Life-Cycle of the Skate (Raja b'ltis), 

 illustrating the union of egg and sperm, E. and S., to form 

 the zygote, Z., the origin of the phorozoon, or larva, or asexual 

 generation, the germinal track from Z. to U.K.Z., which is the 

 primitive germ-cell. The division of the primitive germ-cell is 

 carried to six mitoses, giving 64: primary germ-cells, P.G.C., 

 instead of the full number of 9 divisions in a female skate, 

 yielding .512 primary germ-cells Diagrammatically the evolu- 

 tion of one primary germ-cell, the o7th, is depicted as forming 

 the embryo or gametozoon. To complete the track of heredity 

 from generation to generation through the morphological con- 

 tinuity of the germ-cells — to the 5.3th primary germ-cell a 

 diagram of oogenesis, with the formation of male and of female- 

 eggs, and of spermatogenesis (as in Palndina after Meves" 

 work) to the 20th germ-cell, have been added. In the latter the 

 formation of the ordinaiy spermatozoa, H.S., and of the non- 

 functional worm-like ones. AY.S., ai-e shown. N.B. — In the 

 diagram of oogenesis the "ripening"' of a female-egg is indicated 

 by the larger, and that of a male-egg by the smaller, oocyte 

 and products. 

 Fig. 2. A portion of E. B. Wilson s diagram of the egg-cleavage of 



ISercis. 

 Fig. 3. The egg-cleavage of Nereis^ depicted in Fig. 2, represented 

 after the fashion of Fig. 1. 



^ Were proof wanting of the application of the results of the present 

 research even to the highest animals, it might be found in Hubrecht's 

 remarkable researches into the early development of Tupaja javanica. 

 (A. A. W. Hubrecht, " Die Phylogenese des Amnions und die Bedeu- 

 tung des Trophoblastes," Amsterdam, 1895.) Here the first products 

 of the egg-cleavage are a small number of cells, forming a sac, the 

 trophoblast, and containing one central cell, out of which the entire 

 embryo arises. 



As is now well known, Hubrecht homologises the trophoblast with 

 the larval skin of an Amphibian. It is by no means a new idea to the 

 writer that the trophoblast represents the whole or the greater part 

 of the asexual generation in mammals. The sinj^de clear cell in the 

 sac in Tupaja must either be the primitive germ-cell, which must give 

 rise not only to the embryo, but also to the sexual products, or it 

 must become the primitive germ-cell after one or two additional 

 mitoses. It may be regarded as eloquent testimony of the correctness 

 of my conclusions, that in I'apaJ", Hubrecht should have found the 

 very things which might have been postulated. 



