230 



wert der von Retzius ^) erlangte Befund beina Studium der Endungen 

 der sensitiven Nerven in der Kopfhaut von Lacerta agilis. Die Figuren 

 von Retzius erinnern an das eigentümliche Aussehen der von mir im 

 präokulären Meniscus gefundenen Endigungen. 



Nachdruck verboten. 



Bilateral Symmetry in the Egg of Necturus. 



By Albeet C Eycleshymer, 



Professor of Anatomy, St. Louis University, St. Louis, Mo. 



With 47 Figures. 



During the past decade many observations and experiments have 

 been made on fish and amphibian eggs for the purpose of ascertaining 

 how early in development the vertebrate egg becomes bilaterally sym- 

 metrical. 



Many and varied have been the conclusions. Some claim bilater- 

 alism for the primitive ovum. Others hold that this condition is not 

 present from the first, but originates at some later period. This period 

 may precede or follow the deposition of the egg. Those who regard 

 the egg as bilaterally symmetrical before deposition, claim that this 

 is manifested either through an excentric pigmentation, or an excentric 

 position of the germinal vesicle. Those who regard it as fixed after 

 deposition are not in accord. By some the path of the spermatozoon 

 is considered as the determining factor, by others the first or second 

 cleavage groove, and by still others areas of accelerated segmentation. 



For the purpose of obtaining further data regarding the origin 

 of bilateral symmetry, the writer in 1898 made a series of observations 

 on the eggs of Necturus, and in 1901 they were confirmed and ex- 

 tended 2). 



The Position of natural Markings. 

 The egg of Necturus, owing to its large size, slow cleavage and 

 meroblastic tendency, is an exceedingly favorable object for the study 

 of the relations of cleavage grooves to embryonic axes. The cleavage 



1) Retzius, Ueber die sensiblen Nervenendigungen in den Epithelien 

 bei den Wirbeltieren. Biologische Untersuchungen, Bd. 4, p. 37). 



2) The writer has not appended a bibliography since most of the 

 papers referred to are included in Sobotta's list of citations on „Die 

 Furchung des Wirbeltiereies". Ergebnisse d. Anat. u. Entwickelungsg., 

 Bd. 6, 1896, p. 493 — 589. Those papers not included in his list are 

 cited in the text. 



