EM 13 RYOLOGY AND ASEXUAL REPRODUCTION. Ech. 35 
their traditional relations of position. An eight- or fourteen-cell stage 
often resulted from one of the two first blastomeres, exactly correspond- 
ing to a lateral half of normal sixteen- or twenty-eight-cell stage. This 
contradicts the idea that the blastomeres in a normal or abnormal ovum 
take up a position of mechanical equilibrium. The same result takes 
place on destroying two of the first blastomeres, and it is a matter of 
indifference whether the cells destroyed are both descendants of the 
same cell, or each descendants of different cells. On the other hand, tho 
eight first blastomeres are only equivalent to one another externally, and 
not in their contents, i.e., in the developmental value. If four are left 
uninjured, the groups of eight resulting from them are different in 
different cases. 
In a few instances half embryos were obtained by destroying one of 
the two first blastomeres. Three were half blastulas, and two perhaps 
half gastrulas. Two of the half blastulas were obtained by the pricking 
method, and were hollow hemispheres. In the third case a blastomere 
was killed by shaking, and the resulting blastula consisted of two hemi- 
spheres, one hollow the other solid ; the former had a normal appearance, 
the latter apparently consisted of a granular mass, the remains of the 
dead blastomere, covered over by a cell layer continuous with the normal 
half. [Cf. Driescii.] 
According to Field, in Asterias vulgaris the formation of mesenchyme 
precedes, and is continued during, the process of invagination ; the cells 
arise from the endodermal region of the blastula, and any endodermal 
cell may by division, and probably also without division, become a 
mesenchyme cell. The cells apply themselves to the wall of the gut 
and of the body, and on the oesophagus they form a distinct muscula- 
ture. The connection between the preoral and adoral ciliary bands is 
secondary, not primary, as Semon supposed. The apex of the preoral 
lobe has an ectodermal thickening, corresponding in position with the 
apical plate of Tomaria , but without the pigment spots of the latter. 
Each enterocoele sends up from its dorsal wall a hollow cylindrical pro- 
tuberance, which meets a solid plug of the dorsal ectoderm, thus forming 
a right and a left dorsal pore and pore canal. The right pore and canal 
soon, however, become obliterated. The two enterocoeles then elongate 
anteriorly, and meet and unite in the dorsal lobe. The left enterocoele 
becomes divided in two by a constriction just behind the pore canal. 
The posterior part of tho anterior enterocoele dev elopes five lobes and 
becomes the hydrocoele, which does not separate from the anterior 
enterocoele. 
The bilateral larval form of Echinoderms is ancestral, and not second- 
arily acquired. The paired water pores are homologous in their mode of 
origin, and probably in function, with nephridia. 
Fol has studied the fertilization of the ovum in Asterias. Shortly 
after the entrance of the spermatozoon, a small body, “the spermo- 
centre,” detaches itself from it, after which the male pronucleus swells 
and approaches tho female pronucleus. The latter has its “ ovooentre,” 
