68 FRANKLIN P. REAGAN 



shown in this section. The stage of operation in this case was 

 considerably before the time of appearance of intersomitic 

 grooves. There is as yet no formation of vascular tissues, even 

 in the area opaca. The mesoderm is still uncleft. This and 

 the foregoing case illustrate very well the advantages of remov- 

 ing the outljdng blastoderm at the time of operation. One 

 could, with impunity, have allowed this embryo (Type I, No. 

 122) to incubate longer, provided the blastoderm had been 

 removed to check the result. I have in no case offered as proof, 

 however, any material in which the vascular tissue of the yolk- 

 sac was seen to have come near the line of longitudinal incision. 



Figures 29, 35, and 36 are also from completely isolated frag- 

 ments, illustrating in a positive manner the local formation of 

 endothelium. But in addition to this they display certain 

 special features of interest in other connections to be dealt with 

 later in this work. 



c. Incomplete isolation of meroplasts. Another type of experi- 

 ment performed was one which may be designated as Type II. 

 In such experiments the operated tissue was not completely 

 severed from the outlying blastoderm. Generally the embryonic 

 tissue left to further incubation consisted of a portion of the 

 body axis united with the opaque area anteriorly by a small 

 strip of proamnion. It was repeatedly observed that any con- 

 nection with the blastoderm greatly favored the vitality of the 

 meroplast. Again a consultation of figure 1 will make clear 

 the method of operation. Longitudinal incisions corresponding 

 to A-B and C-D were made of lengths varying from those which 

 separated only a portion of the head, to those which extended 

 back well into the posterior region. In case only the head or 

 trunk-region was desired, an incision G-H served to separate the 

 meroplast posteriorly, and also to separate the posterior blasto- 

 derm from the more anterior portion of the body-axis. The 

 blastoderm not included in the meroplast was removed at the 

 time of operation to be sectioned. 



In meroplast No. 3 of Type II, the longitudinal incisions 

 reached posteriorly to the plane of the Une G-H, which also 

 shows approximately the position of the transverse incision 



