244 bulletin: museum of compaeatiye zoology. 



formation of the three brain vesicles, and long after the segmentation of 

 the mesoderm" ! 



McClure ('89, p. 435, and '90, p. 37) concluded, from studies on em- 

 bryos of Amblystoma, Auolis, and chick, " that the symmetrical con- 

 strictions or folds found in the lateral walls of the embryonic brain are 

 remains of the primitive segmentation of the neural tube, in part atavis- 

 tic, extending [from the spinal cord region] into the primary forebrain." 

 The serial homology of the segments of the myelon and the encephalon 

 seemed to him certain, since he found both the structural characteristics 

 and the nerve relations to be the same in the two regions. " The dorsal 

 roots of spinal nerves take their origin from the apex of their respective 

 myelomeres in exactly the same manner as the nerves of the medulla 

 do from their respective encephalomeres " ('89, p. 437). 



In the same year Miss Piatt ('89) also advocated the view that there 

 is a serial homology between the encephalomeres and the segments of 

 the spinal cord. While she agrees with Orr and Beraneck in regard to 

 the number and appearance of the neuromeres and the ultimate rela- 

 tions of the nerves, she finds that the cranial nerves develop from the 

 constrictions between neuromeres, precisely as the spinal nerves do. In 

 answer to objections to the attempted homology between cranial and 

 spinal segments, she says that in both head and trunk the segmentation 

 is transitory, and that in both regions it is more manifest in the ventral 

 portion of the neural tube. 



The conclusions of Waters ('91) are largely confirmatory of those of 

 McClure, viz. that there is a similar segmentation in brain and spinal 

 cord, with similar sensor nerve relations in both these regions. 



Zimmermann ('91), as a result of his studies on rabbit, chick, and 

 Squalus embryos, thinks he is able to confirm Kupffer's discovery of 

 eight primary cephalic segments or "encephalomeres," although his 

 eight " primare Abschnitte " include forebrain, midbrain, and hindbrain 

 regions, while Kupffer's theoretical conclusion was that his eight "pri- 

 mare Medullarfalten " do not include the hindbrain.^ Although Zim- 

 mermann states that the spinal cord does not appear segmented, he 

 finds in later stages thirteen homodynamous " encephalomeres," and has 

 given a table of these with their nerve relations. He supposes three 

 roots, a dorsal, a lateral, and a ventral one, to be related to each en- 

 cephalomere, but his table gives chiefly the impression of numerous gaps 

 to be filled with hypothetical nerve roots. 



Herrick ('92) states that he finds the segmentation of the medulla 

 1 At first Kupffer thought they did not include the forebrain ! 



