40 



j. F. MCCLENDON. 



ci 



dried them 



no sections came 



again. 



After this treatment 

 off unless they were 



left too long in absolute alcohol. 



HISTORICAL. 



As early as 1870, Metschnikoff repre- 

 sented the neuromeres as paired thicken- 

 ings of the ectoderm on surface views of 

 the scorpion embryo, there being one neu- 

 romere for the segment bearing the eyes, 

 one for the segment bearing the chelicera 

 and so on for succeeding segments back 

 to the point where the fold of the post-ab- 

 domen hid them from view. A " long- 

 itudinal furrow ' runs from the mouth 

 backward, separating the two halves of 

 each neuromere. His figures show also 

 the relation of the anterior neuromeres 

 to the mouth --that is to say the mouth 

 is formed in the first segment and during 

 development moves to the posterior part 

 of the second segment, by which process 

 the second neuromere becomes pre-oral. 



Kowalevsky and Schulgin (1886) de- 

 scribed the transformation of undifferen- 

 tiated ectoderm into nervous tissue. The 

 ectoderm, in the regions of the future 



o 



eanglia, begins to thicken and the cells 



o o o 



to increase in number rapidly and minute 

 pits are formed all over its free surface. 

 These pits are gradually filled up by the 

 growth of the cells forming their walls. 

 This method of cell increase and growth 

 was supposed by the writers to possess 

 peculiar advantages. Unfortunately they 

 gave no figures. 



Saint-Remy made a comparative study of the " brains " of differ- 

 ent groups of air-breathing arthropods, which he published in 



FIG. 2. Second stage of 

 same, X 45- The post-ab- 

 domen is turned under to 

 show the posterior neurom- 

 eres. a&- ~, first to seventh 

 abdominal appendages; ci ! , 

 first cerebral invagination, 

 to join later with its fellow 

 of the opposite side to form 

 the semi-lunar lobe ; ci 2 , 

 second cerebral invagina- 

 tion ; 1 u , first to four- 

 teenth neuromeres. 



