74 PROCEEDIiNGS OF THE AMERICAN ACADEMY 



notch " which is sometimes seen in the blastoderm of the chick. The 

 objections to the concrescence tlieory have been considered by one of 

 us elsewhere : they are mainly drawn from those forms in which the 

 evidences of concrescence have been partially obliterated, or more or 

 less completely disguised. In the case of the teleostei, it is well 

 known that the entire ring is converted into the embryo ; but the 

 manner in which this is accomplished has been very differently under- 

 stood by different embryologists. The posterior end of the embryo 

 is regarded by Oellacher as a fixed point throughout, from which the 

 embryo lengthens forward ; while His, on the contrary, holds that the 

 anterior end of the embryo represents rather the " fixed point" from 

 which the embryo lengthens backward, by the concrescence of the two 

 lateral halves of the embryonic ring. It appears quite certain to us 

 that the principle of concrescence underlies the formation of the 

 embryo. The concrescence appears under the disguised form of a 

 migratory movement of the cells, which accompanies the epibolic 

 growth of the blastoderm. The direction of the movement of the 

 cells composing the ring is that of concresceut growth. We have 

 obtained two embryos showing a very well marked marginal notch at 

 the posterior end of the embryo, in the place of the usually single 

 caudal lobe. This notch, in one case, lasted for more than an hour, 

 but was eventually obliterated. In the case of Elecate, Ryder has 

 stated that the metameric segmentation extends beyond the embryo to 

 the ring itself, which appears to give very conclusive evidence of con- 

 crescent growth. 



IVie Relation of the Median Plane of the Embryo to the First Plane 

 of Cleavage. — We have found it rather difficult, except in a few un- 

 usually favorable instances, to determine the relation of the first 

 cleavage-plane to the median j)lane of the embryo ; but we have very 

 satisfactory grounds for the conclusion that the two planes coincide. 

 Roux and PHiiijer came to the same conclusion in the case of the frog; 

 while Rauber was led to think that the two planes cut each other at 

 right angles. This coincidence appears to hold true in the case of 

 Rliabditis, according to Giitte's figures. In a paper that has just come 

 to hand, E. van Beneden ^'^ makes the following remark on this j)oint : 

 '' Le fait que chez les Ascidiens et probablement aussi chez d'autres 

 animaux a symetrie bilaterale, le plan median du corps de I'animal 

 futur so marque des le debut de la segmentation justifie pleinement 



"^ E. van Beneden. " liecherches sur la Fecondation." Arch, de Biol., IV., 

 fas. 3, p. 570. 



