EMBRYOLOGY OF CLEPSINE. 259 



the first four embryonic (ectodermic) cells, we find them wedged 

 into a somewhat conical space on the oral pole of the egg, 

 having passed from the globular to a pyramidal form. Is it 

 probable that cells can undergo such pressure and still be liable 

 to injury from contact with the membrane ? Besides, during the 

 cleavage period, the membrane is removed from the oral pole of 

 the egg by a distance equal to several diameters of the polar 

 globules, and could not therefore be supported by these globules. 

 There is no time in the whole history of these corpuscles when 

 they could be said to contribute to the maintenance of the space 

 between the membrane and e^g. That this space increases or 

 diminishes entirely independently of these " elastic balls,^'' is well 

 attested by the fact that during the elimination of the first polar 

 globule it is sometimes present on both poles of the egg, and by 

 the fact that this globule after its expulsion, is thrust back into 

 the yolk till it is quite out of sight. Biitschli (o-W) has called 

 atlention to the fact, that in Paludiiia and Neritina, and in all 

 cases where eggs unprovided with membrane swim about in the 

 fluid contained in the cocoon, the polar corpuscles could afford 

 no protection to the embryo. The same is true of many eggs 

 which are not laid in cocoons (Echinus and some Coelenterates) , 

 and of all eggs where the membrane stands from the outset at 

 a great distance from the yolk (many Molluscs). 



Biitschli (a-rl^TT* Wy) comparing the process of fecundation 

 with the conjugation of Infusoria, claims for the polar globules 

 a ])ali7igenetic origin. In some cases (Vorticella, Stylonychia) 

 conjugation results in a complete coalescence (copulation, En- 

 gelmann) of two unicellular individuals (male and female) ; but 

 in most cases the conjugating individuals may be regarded as 

 hermaphrodite (0. Hertwig -^^, Engelmann -j^Wj r5utschli ijVtr)^ 

 and during their temporary union a reciprocal fecundation takes 

 place, the "nucleolus-segments^' with a little protoplasm being 

 interchanged (according to Engelmann). Biitschli (^) has 

 observed this interchange of nucleoli in Paramoecium Bursaria 

 and P. putrinum. Both Hertwig and Engelmann regard the 

 nucleolus as a male element and the nucleus as a female element ; 

 and Hertwig sees in the pronuclear-stage of the egg a repetition 

 of the hermaphrodite condition of the Infusoria. These 

 authors differ widely in regard to the morphological value of the 

 nucleolus. Hertwig compares it with the Sperraakern, and the 

 nucleus with the Eikern ; Engelmann, on the other hand, says 

 that "the nucleus j>j/«5 the nucleolus is homologous with the 

 ordinary cell-nucleus." Biitschli {-^^tj tt)^ ^l^o first advanced 

 the idea of the nuclear character of the " nucleolus," admits 

 (tsV) the plausibility of Hertwig's comparison, provided the 

 exchange of nucleoli be an event of normal occurrence ; but 



