48 Oen. Sub. 
X. GENERAL SUBJECTS. 
pressure in embryo Fundulus. Tiger-like pigmented marking of yolk-sac 
depends on the distribution of blood-vessels ; Loeb (321). 
Experimental embryology : Summary and criticism ; Roux (434, 435) : 
summary of researches ; Thomson (503). — Summary of facts as to post- 
generation, &c. ; Barfurth (23). 
Halves or half-sized wholes ? Denial of Chabry’s conclusion that after 
the destruction of one of the two first blastomeres of the Ascidian ovum, 
the result is a complete embryo of half size. A typical half-morula, half- 
gastrula, half- larva, is formed. Note of a half-embryo of Siredon pisci- 
formis ; Barfurth (26). — Experiments on Amphibian ova ; Barfurth 
(24, 25). 
Cleavage, effect of pressure, &c. ; Born ( 52). — Experiments with 
electric currents, &c. ; Roux (439). 
Warmth affects the blastulie of Spha-rechinus granularis , so that exo- 
gastrula and anenteria result. Dilution of the sea-water induces 
variations in the formation of micromeres, and yet normal Plutei may 
result. Removal of the micromeres of the “vegetative” half, of the 
“ animal ’’ half, &c., does not prevent the development of a normal 
Pluteus. Therefore the blastomeres are equivalent, and organogenesis is 
induced by differences of position and by external stimuli acting on the 
self-differentiating plasma ; Driesoii (135). 
Effect of pressure : Effect of pressure of a cover-glass on ova of Sea- 
Urchins from which the membranes are shaken off. The eight-cell stage 
consists of an annular zone of “ animal ” cells, and two separate zones, 
one at each pole, of “ vegetative ” cells. Yet normal Plutei result : thus, 
a blastomere may give rise to something quite different from that to 
which it would have given rise in normal development ; some cells, 
which would normally form endoderm do in reality form ectoderm j 
Driesoii (136). 
Criticism of Driesch : It is an unproved assumption that the eight 
Echinoid blastomeres formed, under pressure, in Driesch’s experiment are 
the equivalents of the cells in the normal 8-cell stage. Driesch’s experi- 
ment shows only that segmentation may be greatly altered without 
affecting ultimate development, and if one of the first two blastomeres 
form a complete gastrula, this is simply a striking instance of regenera- 
tion ; Braem (56). 
Driesch’s argument that because a normal Pluteus larva is formed after 
disarrangement of blastomeres, or removal of some of them, therefore the 
blastomeres are mutually equivalent, is fallacious. Roux explains the 
normal result as due to the residual powers of regeneration and post- 
generation. Roux recognizes two modes of development : the normal 
or direct, in which there is a typical system of definitely-determined 
processes ; and the indirect or atypical, in which the post-generative 
processes are at work — when, for instance, the half-embryo of a frog 
forms a whofe embryo. He distinguishes the idioplasms of direct and 
indirect development ; Roux (434, 435, 436). 
Effects of lithium salts on the development of Echinoid embryos : The 
