1034 ELLIOT ROWLAND DOWNING 
THE PRIMITIVE ANIMAL TYPE 
Botanists seem agreed that the significance of the persistent 
gametophyte in the higher plants is that it represents a return 
to a primitive type. The conclusion seems a proper one. We 
would similarly expect the gametozoic generation, then, to rep- 
resent a primitive animal type. The spherical spermatophore, 
arising from a spermatogonium as a spore might cleave, must 
find its counterpart in the phylogeny of the higher animals as the 
thallus-like gametophyte of the higher plants indicates a thal- 
line ancestry for them. 
Possibly the Volvocales approximate most closely to such an 
hypothetical ancestral form. The group is one that has been 
appropriated by many zoologists as showing marked animal 
affinities, and is admittedly a widely aberrant type by all botan- 
ists. Volvox reproduces asexually; certain cells of the central 
cavity, the so-called parthenogonidia, reproduce by fission, and 
the offspring divide and subdivide, the products cohering in spher- 
ical masses to form colonies like the parent. These are freed 
when the parent colony disintegrates. Volvox also reproduces 
sexually. Certain cells differentiate as eggs and come to lie in 
the interior of the colony. Other cells, similarly discharged to 
the interior, develop within them numerous sperm. The sperm 
break out of their containing cells and fertilize the eggs. The 
oospore remains inactive for some time as a resting spore but 
finally develops a colony like the parent. My preparations are 
not yet sufficiently clear to permit of working out all the chro- 
matin changes, but lam quite positive that reduction occurs in Vol- 
vox, 4s it does in animals, during gametogenesis. 
In the Dicyemidae, looked upon by many, zoologists as the 
connecting link between the protozoa and the metazoa, the proc- 
ess of reproduction is suggestively similar, except that the asex- 
ually produced young bore out from the parent to an independent 
existence instead of awaiting the death of the parent. The center 
of the dicyemid is occupied by a cell, in which, besides the nucleus, 
there are one to several embryo cells from which the asexual 
individuals arise. These cells are evidently the homologues of 
