64 FRANKLIN P. REAGAN 



embrj^onic axis (fig. 1, A-B and C-D). Next, a transverse inci- 

 sion (E-F) served to sever the anterior portion of the body 

 from the more posterior portion; this incision was usually made 

 at the anterior intestinal portal. At this time, before com- 

 pletely separating the head-fragment from the small band of 

 tissue which held it anteriorly, it was found advisable to remove 

 the main portion of the blastoderm. The method of doing this 

 will be understood by following the broken curved lines from 

 points B and D, laterally and posteriorly; a transverse incision 

 was then made, joining their posterior extremities. The large 

 piece of blastoderm obtained by this means was preserved at 

 once, to be sectioned later. The head-fragment was then com- 

 pletely isolated by the incision shown in figure 2. This figure 

 represents a side view of the head (represented entirely too large) 

 which has been pushed back so that the incision can be made 

 with greater ease. Such a fragment generally sinks to -the 

 bottom of the sub-germinal cavity. Following the operation 

 the egg was sealed and incubated to a total age of from twenty- 

 four to forty-eight hours; after that time, degenerative changes 

 could be seen in the tissues. Differentiation did not usually 

 proceed beyond that of a normal thirty-two hour embryo. 

 Growth, on the other hand, varied greatly. Meroplasts equally 

 differentiated might vary greatly in size. As I have previously 

 noted (52, p. 334) it seems that the embryonic meroplast '^ pos- 

 sesses an inherent capacity for differentiation which tides it over 

 to a time when heart-pulsation should normally provide a means 

 of tissue-respiration." 



After the final incubation the tissue-fragment was removed 

 and preserved in picro-acetic acid, sectioned, and stained. The 

 stain ordinarily employed was my modification (51) of Mann's 

 methyl blue-eosin stain. In a number of cases in which an 

 attempt was made to utilize mitotic figures as a means of deter- 

 mining the origin of the prevascular tissue, the sections were 

 stained in iron-hematoxylin and counterstained with acidulated 

 methyl blue. Ordinarily the sections were cut 4 micra in 

 thickness. 



