STRUCTURE AND DEVELOPMENTAL RATE 243 



usual single axis does not arise with sufficient influence to suppress 

 the origin of other axes, and twins or double individuals result. 

 Twins and double monsters are, therefore, types of developmental 

 arrests. 



2. The suppression of the eyes or modification of their struc- 

 ture : When an arrest is induced later than the above, but before 

 the origin of the embryonic shield in the Fundulus embryo, all 

 known modifications in the structure of the eyes may result in 

 otherwise normal individuals or in further deformed specimens. 



3. Suppression of the primary brain ventricles inducing sub- 

 sequently deformed or tubular brains: Arrests induced before 

 and just about the time of first appearance of the embryonic 

 shield result in malformation of the embryonic brain ventricles. 

 These do not express their usual bilateral outgrowths and are 

 frequently of a simple tubular outline. 



The periods of arrest necessary to induce the eye and the brain 

 modifications are so close together or so nearly the same, that 

 one generally finds combinations and mixtures of the defects 

 among the same experimental group of embryos. Arrests at the 

 earlier moment give a majority of eye conditions, many without 

 brain involvements, while arrests at the slightly later stage give a 

 majority of brain modifications, a few with fairly well-formed eyes. 

 The individual variations in developmental moments among the 

 embryos of a group also tend to contaminate the results and give 

 mixtures of the two classes of deformities. 



4. Modified or contorted mouth and branchial systems: Ar- 

 rests during the embryonic shield stage, and earlier, frequently 

 cause deformities of the mouth and branchial regions of the Fun- 

 dulus embryo. In a few cases these deformities have existed in 

 individuals otherwise normal. They, therefore, must possess a 

 critical moment occurring at a time more or less distinct from 

 that of the other organs. The close association of mouth abnor- 

 malities with those of the eyes is probably not due to identical 

 critical moments of origin in the two cases, but more likely to 

 the fact that when a slow rate of development exists during the 

 eye moment it is rarely completely overcome and the normal 

 rate reestablished before the critical moment for the bilateral 

 outgrowths of the mandibular pouch is reached. 



