28 CHARLES R. ESSICK 



the migrating strands of nuclear material were obtained in em- 

 bryos stained in methylene blue. After previous hardening 

 (10 per cent formalin is excellent), the brain is carefully taken out 

 of the skull and all of the pia mater dissected off; very great care 

 must be exercised in removing this vascular membrane in order 

 that the tiny penetrating vessels do not tear the surface of the 

 brain which then stains very deeply along the ruptured edges. 

 The specimen is placed into an aqueous solution of methylene 

 blue (saturate aqueous methylene blue and water equal parts) 

 for thirty to sixty seconds, rinsed in water, and transferred to water 

 for study. The whole brain is tinted blue but the most prominent 

 parts take a more intense stain so that all of the fine surface 

 irregularities are outlined in great detail. This brings out with 

 remarkable clearness the anastomosing strands of cells converging 

 into the pontine formation. 



Confusion might arise out of the terms employed here so that 

 a word may not be out of place concerning their meaning. Inas- 

 much as the flexures of the brain as well as the position of the head 

 are not fixed, I have used the words 'cephalad' (forward, front, 

 anteriorly, cerebrally), 'caudad' (backward, behind, spinalward), 

 'dorsal,' 'ventral,' 'lateral' and 'mesial' just as if the central nerv- 

 ous system were a simple straight tube placed in the head as the 

 spinal cord is in the body. This, it seems to me, will facilitate 

 the description of relations of parts which are constantly shifting 

 their positions in relation to the body. In addition, I might state 

 that I have used the term 'neuroblast' loosely, so as to include all 

 undifferentiated cells which have not taken on a definite form. 



In considering the development of these basilar masses it may 

 be of advantage to review briefly some of the relations which exist 

 in the rhombencephalon just before the cells, destined for the 

 pontine and arcuate nuclei, set out from their germ centers. His 

 ('91) has carefully reconstructed some of the intramedullary 

 nuclei and nerve roots with their relations to the surface and brain 

 flexures in an embryo of 22 mm. (figs. 5 and 17). He has called 

 attention to the fact that at this time, towards the end of the 

 second month, the formation of new neuroblasts has ceased in the 

 medulla and it is only with difficulty that a mitotic figure is dis- 



