THE BLASTULA IN RELATION TO TWINNING 



381 



(3) an organization center or the ability to develop such a center must 

 be present in order that the various organs may be integrated into 

 an harmonious whole; and 



(4) the ability or faculty for regulation, that is, the power to rearrange 

 materials as well as to reproduce and compensate for the loss of sub- 

 stance, must be present. 



3. Some Experimentally Produced, Twinning Conditions 



The isolation of the first two blastomeres in the sea-urchin egg and in 

 Amphioxus with the production of complete embryos from each blastomere 



TRANSPLANTED PROSPECTIVE- 



EPIDERMAL ECTODERM 



Fig. 183. Early gastrula of darkly pigmented Triton taeniatus with a small piece of 

 presumptive ectoderm of T. cristatus lightly pigmented inserted into the presumptive, 

 neural plate area shown in (A). (B) Later stage of development. (C) Cross section of the 

 later embryo. The lighter eye region shown to the right was derived from the original 

 implant from T. cristatus. (After Spemann, '38.) 



Fig. 184. Demonstration that the presence of the organizer region or organization 

 center is necessary for development. (Redrawn from Spemann, '38.) (A) Hair-loop 

 constriction isolates the organizer areas in the dorsal portion of the early gastrula. (B) 

 Later development of the dorsal portion isolated in (A). (C) Later development of 

 ventral portion of gastrula isolated in (A). (D) Constriction of organizer area of early 

 gastrula into two halves. (E) Result of constriction made in (D). Constrictions were 

 made at 2-cell stage. 



