WEYSSE. — BLASTODERMIC VESICLE OF SUS SCROPA. 303 



blastodermic vesicle, and to the theoretical interpretation of some of 

 the phenomena which occur in the embryo of the pig, as I have 

 described them. 



III. Historical and Theoretical. 



1. Consideration of Observations on the Blastodermic Vesicle in 



General. 



On account of the incompleteness of our knowledge of the facts con- 

 cerning the blastodermic vesicle of the Mammalia, there are naturally 

 several theories with regard to the exact method of its formation, and 

 the interpretation of the vesicle when once it has been formed. Since 

 my own material begins with the completely formed blastodermic 

 vesicle, I cannot from an actual observation of the process of develop- 

 ment add anything to our knowledge of the method of it formation ; 

 but the phenomena which the vesicle at this stage presents, taken in 

 connection with the observations of other investigators on mammalian 

 embryology, serve to throw not a little light on several of the mooted 

 points of its development. 



Although the accounts of mammalian cleavage are few, and not in 

 accord with one another, it seems to be pretty well established that 

 cleavage results in the formation of a hollow sphere of cells, contain- 

 ing on the inside, at one pole, a more or less irregular mass of cells. 

 These facts have been recorded by various authors ; as, for example, 

 Lieberkiihn ('79), Van Beneden ('80), Van Beneden et Julin ('80), 

 Heape ('83), Hubrecht ('90), Duval ('91), Robinson ('92), Christian! 

 ('92), and others. The method of formation of the germinal layers 

 from these structures is in much dispute, however, I will consider 

 briefly four theories which have been advanced on this subject, upon 

 which my own investigations seem to throw some light ; but I would 

 not be understood as trying to make all the observed methods of 

 mammalian development conform to one type, — there certainly are 

 not as yet sufficient data for that ; and besides, many reasons exist 

 for supposing that there may be several types of development, con- 

 forming to the varied conditions under which the very young embryo 

 is placed in different mammals. 



The first theory is that which Van Beneden ('80) formulated for 

 th« rabbit. He found a well defined outer layer of cells, just beneath 

 this in the region of the germinal disk a layer of flattened cells 

 lining a limited area, and within this a layer which had extended 

 partially around the inner wall of the outer layer, and these three 



