STEUCTURE AND DEVELOPMENTAL RATE 261 



13. The initial growth giving origin to an embryonic system, 

 such as the brain and spinal cord, is linear in type, until a definite 

 length is attained when linear growth subsides. This is followed 

 by a series of lateral outgrowths in consecutive fashion. These 

 lateral outgrowths from the central nervous system may be 

 experimentally suppressed by slowing development at definite 

 times, and when all are absent a simple tubular brain is the end 

 result. The same plan of development holds for the foregut and 

 its lateral outgrowths to form the mandibular pouch, etc., and 

 the development of this system may also be modified in a manner 

 similar to that mentioned for the brain. 



14. Monstra in defectu and rnonstra in excessu, which have 

 frequently been treated as such distinctly different classes of con- 

 ditions, are as a matter of fact closely similar. Both classes of 

 anomalies are due to a common cause and may actually both 

 exist in the same specunen. For example, an arrest of develop- 

 ment before gastrulation may cause a blastoderm to form two 

 embryonic processes which later develop into a double-headed 

 individual — a typical monstrum in excessu. At a very early stage 

 one of these embryonic processes may become inhibited and later 

 form a cyclopean eye instead of the usual two lateral eyes; this 

 head is then a typical case of monstrum in defectu. The fact 

 that the normal individual stands between these two arbitrary 

 classes of monsters has no other significance than that the mon- 

 sters themselves are simply modifications of the normal condi- 

 tion resulting from an unusual reduction in the rate of develop- 

 ment during certain critical periods. 



15. The great importance of developmental rate in influencing 

 the type and quality of structure is not confined solely to embry- 

 onic development, but postnatal development, and structures are 

 similarly influenced by the rate at which the processes are accom- 

 plished. This phase of the subject is to be presented in a sub- 

 sequent communication. 



16. In view of experimental results, it becomes evident that 

 normal development of the vertebrate embryo depends acutely 

 upon the stability of certain factors in the environment. Changes 

 in the conditions of moisture, temperature, and oxygen supply 



