97 



Nachdruck verboten. 



The Rhythm of Reproduction in Mammalia. 



By J. Beard, University of Edinburgh. 



Under this title, which forms the heading of the closing chapter 

 of a larger work, it is proposed to give a concise summary of the 

 conclusions arrived at in the latter. The complete memoir is now in 

 the press, and will shortly be published ' ) , uniform with "Certain 

 Problems of Vertebrate Embryology", Jena 1896. The newer work 

 arose out of the latter, and forms its natural sequel. 



The new inquiry begins with an examination of the conditions, 

 under which reproduction can be, and is, effected, when uterine de- 

 velopment and nutrition of offspring by means of mammary glands are 

 initiated. In the memoir ^) of last year it was for the first time 

 established, that there is in the development of any Vertebrate a 

 period or phase termed the critical one. It is there defined thus: — 

 "there is a stage (better a phase) in the development of every Verte- 

 brate embryo, during which, and only then, it resembles the embryo 

 of any other Vertebrate in a corresponding stage (phase) in certain 

 general features. But, while it thus agrees with any other embryo in 

 this stage (phase) in characters, which are common to all Vertebrate 

 animals, it differs from the embryo of any other class in certain special 

 class features, and also from any other embryo of the same class, but 

 of a different order, in other and ordinal characters. Immediately 

 before this stage (phase) is reached, it begins to put on generic and 

 specific characters, and, thus, it then begins to differ from all other 

 embryos in these." 



It was also shown, that at this phase, if there was a yolk-sac filled 

 with yolk, the embryo began to annex its contents, and that, if with 

 uterine development the yolk of the yolk-sac had become a negative 



1) As "The Span of Gestation and the Cause of Birth. A Study 

 of the Critical Period and its Effects in Mammalia". Jena, Gustav 

 Fischer, 1897. 



2) J. Beard, On certain Problems of Vertebrate Embryology. Jena, 

 Gustav Fischer, 1896. 



