384 MABEL BISHOP 



(Mubryo and it has lioon p;ivon special attoiition. It consists of 

 the joined left half of head A and the right half of head B and is, 

 therefore, a niandibuloniaxillary complex with contributions from 

 both heads. Since its structures are intimately concerned with 

 the identification of the median isolated ganglion, its internal 

 anatomy is discussed in that connection (p. 411). Throughout 

 the embryo the duplicity of the teras is more apparent internally 

 and greatest in the head region. 



METHOD OF STUDY AND TERMINOLOGY USED 



A careful study of the anterior five hundred sections has been 

 made, therefore, with special reference to the nervous system in 

 the head and neck. A wax-plate model of the brain, spinal cord, 

 and nerves in these regions was made as follows: Beginning with 

 section 500 and using every alternate section cephalad, all parts 

 of the nervous system visible under low-power objective of the 

 compound microscope were magnified twenty-five diameters and 

 projection lantern drawings made. These were transferred to 

 wax plates 1 mm. in thickness, and the model stacked in seg- 

 ments in such manner that it could be readily assembled. Later 

 it was cut through the median sagittal plane so as to afford a 

 median aspect of the brain and spinal cord (fig. 5). In stacking, 

 the segments were checked up by rereadings of the microscopical 

 sections and each nerve painted a different color. Black plates 

 were used to indicate every tenth section. The study has been 

 further checked by constant reference to serial sections of normal 

 pig embryos loaned by Prof. F. R. Lillie, of the Department of 

 Zoology of the University of Chicago, and by such references in 

 the literature as had any bearing upon the subject. 



Terminology. Before giving a description of the structural 

 conditions found in the teratological embryo, it seems advisable 

 to make clear the terminology used and to call attention to cer- 

 tain points of morphology that are important in interpreting the 

 sections. 



For the sake of simplicity, the two members of the teratological 

 head are designated head A, the right member; head B, the left 

 member. Teratological head includes both members and refers to 



