Literary Notices. xxxix 



Ectodermal Elements in Cranial Ganglia. 1 



Miss Piatt call attention to the relations found in the ganglia 

 of Necturus between mesoderm and ectoderm. She divides the meso- 

 derm into mesectoderm and mesentoderm, according as it is de- 

 rived from one or the other of the two primitive layers. Cells arising 

 from the ectoderm and entoderm respectively are differentiated from 

 each other until a late stage of embryonic development by the yolk 

 globules which they contain. " I have not observed the migration of 

 cells from the brain into the ganglia except through the neural crest, 

 and if such migrations occur at an early stage they are not common. 

 I have moreover seen some of the original mesectoderm cells which 

 take part in the formation of Gasserian ganglion ' spin ' fibres to muscle 

 cells lying in the mandibular mesentoderm. These are undoubtedly 

 motor fibres 'spun' from cells that have not migrated from the brain, 

 unless through the neural crest. Connecting this fact with observa- 

 tions made by Froriep and myself on the development of the trochle- 

 aris in Selachians, I am inclined to think that before the great motor 

 and sensory tracts of the central nervous system are established indi- 

 vidual peripheral cells may complete in themselves a reflex arc, either 

 as an ancestral characteristic repeated in embryonic development or as 

 an advantageous larval acquisition." 



The Optic Yesicles of Elasmobranchs and their Serial Relation to Other 

 Structures on the Cephalic Plate. 2 



Mr. Locy states that the involutions that are to give rise to the 

 optic vesicles appear at a very early stage. As soon as the cephalic 

 plate has been formed, by one expansion of the anterior end of the 

 embryo, two faint circular depressions are to be seen upon its ex- 

 treme anterior surface. These depressions grow deeper and run to- 

 gether in the middle line, and the continuous infolding produced in 

 this way divides the cephalic plate into an anterior depressed region, 

 and a posterior elevated region. The infolding gives rise, also, to 

 infundibulum. The optic vesicles, which are started near the median 

 line, grow outwards laterally, and come to occupy the lateral parts of 

 the depressed region, but as the infolding forming it does not extend 

 entirely across the cephalic plate, the optic vesicles do not reach the 

 margin of the medulary folds. When distinctly formed, the optic 



2 Julia B. Platt. Ontogenetic Differentiations of the Ectoderm in 

 Necturus. Anat. Anz. IX, i, 2. Oct. 1893. 



2 Locy, W. A. Journ. Morphology, IX, I. 



