Maech 27, 1903.] 



SCIENCE. 



489 



bital plate lias a deep pineal pit in its 

 under surface. Two other pits, shallower 

 than the first, are symmetrically placed 

 behind it on the under surface of the semi- 

 circular post-orbital plate. 



4. The lateral eyes in Bothriolepis were 

 placed on short stalks attached to the mar- 

 gin of the orbits by flexible membranes. 

 The lateral end of each stalk was convex, 

 covered with a smooth shell, and could evi- 

 dently be raised above the orbit or lowered 

 into it. 



5. The structure and relations of the 

 'mental plates' of Bothriolepis show that 

 they can not be regarded as either upper or 

 lower jaws of the vertebrate type. If 

 movable at all, they must have moved to 

 and from the median line, bringing their 

 thickened and hent-over median edges into 

 opposition, like the crushing mandibles of 

 an arthropod. 



6. The mouth was very small, round or 

 oval (not a wide transverse opening), lo- 

 cated between or just behind the mental 

 plates. 



7. The so-called 'semilunars' consist of 

 at least three pieces. Their shape and 

 articulating surfaces show that their pos- 

 terior margins were freely movable in a 

 dorso-ventral direction, like an operculum. 



8. Two plates were found supposed to 

 be, one the distal joint, the other a basal 

 plate, of the proximal joint of the pectoral 

 appendage of Tremataspis. 



9. The basal joint of the appendage in 

 Bothriolepis contains a short axial skele- 

 ton whose expanded distal end shows indi- 

 cations of several fin-like rays. 



10. The gill chamber of Bothriolepis is 

 a shallow depression on the dorsal surface 

 of the anterior ventrals. 



11. In one specimen the gill chamber 

 was partly covered by a folded membrane 

 and it contained indications of gills. The 

 most exposed gill was a flattened body of 

 elongated form. It appeared to be jointed. 



with a single broad spur, and a fragment- 

 ary filament, near its base. The end 

 directed toward the base of the pectoral 

 appendage terminated in a leaf-like ex- 

 pansion. 



These facts confirm the author's view 

 that the Ostracoderms can not be classed 

 with the true fishes. 



Maturation Changes in the Egg of an 

 Opisthohranch iefore Deposition: W. 

 M. Smallwood, Syracuse University. 

 (To be published in the Bulletin of the 

 Museum of Comparative Zoology at Har- 

 vard College.) 



Experiments on Merogony in Neniertine 

 Eggs, with Reference to Cleavage and 

 Localization: Edmund B. Wilson, Co- 

 lumbia University. 



The experiments were performed in 

 order to examine the ciuestion of prelocal- 

 ization of the factors determining the 

 cleavage mosaic in the tinsegmented egg. 

 The nemertine egg presents features that 

 allow of its definite orientation from the 

 moment of discharge from the ovary. Egg 

 fragments, obtained before formation of 

 the polar bodies, by shaking the egg to 

 pieces or cutting the eggs individually in 

 various planes with the scalpel, segment 

 exactly like entire eggs of diminished size. 

 Whatever be the plane of section the frag- 

 ments may, if not too small (one fourth 

 the bulk of the egg or larger), give rise to 

 closed blastulas, which may gastrulate 

 normally and produce dwarf pilidia nor- 

 mal except in size. Isolated blastomeres 

 of the two-cell stage may likewise produce 

 perfect pilidia of half the normal size; 

 isolated one fourth blastomeres may pro- 

 duce dwarf pilidia, never entirely normal, 

 but sometimes very nearly so. In either 

 ease the isolated blastomere segments, not 

 like a whole egg, but as if the missing 

 portion of the egg were present. Blastulas 



