626 TEXT-BOOK OF EMBRYOLOGY. 



protoplasm originally occured in several places, each droplet produces an 

 embryo and the result is a triple or quadruple monster (Loeb) . 



Similar experiments have also been performed on Vertebrates. For example, 

 the two primary blastomeres of Amphioxus have been partly separated and 

 double monsters developed. The blastomeres in the four-cell stage have been 

 incompletely separated, resulting in double embryos of equal size, or triple 

 embryos, or quadruple monsters. Frog's eggs have been made to produce 

 double monsters by keeping them turned upside down after the morula stage; 

 the same result has also been produced by loosely tying a ligature in the furrow 

 between the two primary blastomeres. A most curious result has been obtained 

 by splitting the limb bud of a growing tadpole one or more times. Two or even 

 a cluster of limbs may develop where only one does normally (Tornier). 



These few examples from the great number of experiments which have been 

 performed serve to show that great light can be thrown upon the problems of 

 teratogenesis by experimental embryology. While they do not prove that there 

 are no other possible modes of origin for malformations, they indicate the im- 

 portance of external influences upon development, and afford tangible evidence 

 in the study of monsters. 



THE PRODUCTION or MONSTERS IN SINGLE EMBRYOS. In single embryos 

 of the lower forms it is possible to produce by various means a great variety of 

 malformations, many of which are likewise comparable with malformations 

 found in human embryos. By placing recently fertilized eggs of Fundulus 

 in a 1.5 per cent, aqueous solution of potassium chlorid, embryos may be 

 produced in which the heart is developed but does not beat, and in which the 

 blood vessels appear in their normal positions but with irregular lumina (Loeb). 

 After extirpating the heart anlage from very young frog embryos, the latter grow 

 irregularly and become edema tous; the larger vascular trunks are distended, 

 but the capillary system is imperfect or absent, and the development of many 

 other organs is inhibited (Knower.) Similar results may be obtained by 

 placing the young embryos in aceton-chloroform which inhibits the heart 

 action. 



It is possible to produce typical spina bifida in frog embryos by putting 

 them, during the early stages of development, into a 0.6 per cent, solution of 

 sodium chlorid (Morgan and Tsuda). If the eggs of Axolotl are treated with 

 a 0.7 per cent, solution of sodium chlorid all the embryos have spina bifida 

 (Hertwig). If the eggs of Fundulus are placed in a solution of magnesium 

 chlorid, 50 per cent, of them produce embryos with cyclopia (Stockard). 



Even these few examples from the enormous number of experiments that 

 have been tried in the study of single monsters again lead to the conclusion that 

 at least some malformations in single individuals are due to external influences 

 and not to germinal defects. 



