REPRODUCTION. 851 



Through invagination of the blastula, forming the cup-shaped gas- 

 trula, or through delamination from the disklike layer of cells, a 

 blastoderm is gradually formed, which consists of two distinct layers, 

 an outer one, called ectoderm, and an inner one, called entoderm. 



The next step of advancement in the development is one to 

 which the attenfion of all distinguished embryologists has been kept 

 engaged for many decades. It is the gradual formation of a distinct 

 third layer of cells between the outer two, which is called the middle 

 layer, or mesoderm. Some investigators have proved that in certain 

 animals the mesoderm originates from the entoderm; others, again, 

 have shown that it takes its origin from the ectoderm. The present 

 state of our knowledge leads us to assume that from the morpholog- 

 ical point of view throughout the animal kingdom both modes of 

 origin can be found to take place. The far more important phy- 

 siological aspect and significance of the question is closely correlated 

 with the broader questions of general biology, and to them we will 

 turn our attention now. 



With the differentiation of blastoderms into three distinct, so- 

 called germinal layers, the foundation for a physiological division 

 of labor is established and the formation of the various organs 

 organogenesis begins. A description of the details of it, however, 

 are beyond the scope of this text-book. Here it is only necessary 

 to emphasize the important fact that each of the three layers repre- 

 sents not only a morphological, but also a physiological, unit, as each 

 one of them gives rise only to certain tissues and organs of the adult, 

 and neither one can be substituted in that respect by another with- 

 out producing abnormal conditions. This fact is of such great prin- 

 cipal significance that pathologists have adopted a classification of 

 tumors according to the three germinal layers and their derivates. 



An exceedingly important, as well as interesting, question which 

 occupies the minds of modern biologists is whether the physiological 

 differentiation of the germinal layers begins only with the forma- 

 tion of these structural units, or whether it is already present at an 

 earlier stage of the development of the ovum, and becomes more per- 

 ceivable only in the germinal layers. The attempt to answer this ques- 

 tion leads us over to the consideration of the vital problems in 

 general biology evolution and inheritance which I will shortly 

 take up.* 



It has been shown by Loeb that the unfertilized egg of the sea 



* The preceding pages upon reproduction have been contributed by Dr. 

 P. Fischelis. 



