336 JOHN WARREN 



dorsal structures. They are wholly illusory from a morphological 

 standpoint." 



Froriep (31) in mole embryos of 5.5 mm., which corresponded 

 about to a four weeks human embryo, found two neuromeresin 

 the ' Zwischenhirn' and three in the mid brain. In Salamandra 

 maculosa and Triton cristatus he observed the segments seen 

 earlier by Kupffer but differed from that author as to their num- 

 ber. This author is rather sceptical about the morphological 

 value of neuromeres and is inclined to regard them as results of 

 mechanical pressure from the mesoderm. 



Locy (63, 65) in Acanthias observed a very early segmentation 

 of the primitive neural plate upon closure of the neural tube, which 

 appeared along the edges of the plate. He stated that these 

 segments could be traced without interruption through all stages 

 of the open neural plate into those structures that in later periods 

 were called neuromeres. This segmentation extended through 

 all of the brain. Locy found two mid brain segments and three 

 fore brain segments. The fore brain neuromeres were first the 

 olfactory, and second the optic. The optic vesicles appeared 

 ver}^ early on the neural plate and after the neural tube closed 

 they seemed to belong especially to the parencephalic segment 

 of the diencephalon. As regards the third neuromere he thought 

 that its nerve was possibly the nerve to the pineal sense organ 

 as sense organs and cranial nerves undoubtedly at first had definite 

 segmental relations in neural segments. This condition persists 

 in the cord but in the brain the primitive relations are greatly 

 modified or obliterated. He concluded that neuromeric seg- 

 mentation appears long before there is any segmentation in the 

 mesoderm and therefore it is more primitive than mesodermic 

 segmentation. The various segments are serially homologous 

 and related to cranial and spinal nerves. The segments undergo 

 most modification in the fore brain and mid brain where they 

 disappear first. 



Neal (73, 74) made his observations on embryos of Acanthias. 

 He did not agree with Locy that the segments found on the edge 

 of the open neural plate were true neuromeres, because of their 

 irregularity and variation both in size and number. He believed 



