252 WATASE. [Vol. IV. 



and attempts to derive those germ-layers themselves from a 

 simpler cleavage segment or segments ; and instead of compar- 

 ing well-arranged layers in the establishment of homologies of 

 structures in different organisms, it seeks to reduce them to a 

 still simpler aggregation of cells. The teloblasts of Whitman 

 and Wilson, in which an early differentiation of the tissue-germs 

 is recognized, furnish admirable examples of this class of phe- 

 nomena. May not the well-known fact of the early separation 

 of sexual cells in the body of a metazoan embryo belong to the 

 same category of facts .-' 



While the teloblasts represent only a part of the entire organ- 

 ism, as, for example, the nervous system or the excretory organ, 

 the sexual cells contain the " Anlage" of the complete organism. 

 The difference appears to me to be more one of degree than of 

 kind. While the one completes its development during the 

 ontogeny of the organism, the other completes its development 

 at the next generation. 



Although this tendency to recognize early differentiation of 

 parts in an early stage of the dividing ovum is essentially the 

 outcome of more recent studies, it was foreshadowed in the 

 works of several earlier investigators. Von Baer, as far back as 

 1834, recognized the animal and vegetable poles in the frog's 

 ovum. Newport, as far back as 1854, identified by an ingen- 

 ious device the first cleavage plane of the ovum with the 

 median axis of the embryo in the frog, and pointed out the 

 origin of the head and tail end of the embryo from definite 

 parts of the ovum. Pfliiger and Roux have more recently 

 shown the coincidence of the median axis of the embryo with 

 the plane of the first cleavage furrow in the ovum of the frog. 

 The striking series of results brought out by the latter in 

 experimental embryology are the most important contributions 

 in this connection. Mark has discussed the "primitive axis" 

 of the ovum in Limax ; Goette has fully treated the relation of 

 the promorphological axes of the ovum in the annelid, and their 

 relation to the cleavage and early differentiation of tissue-cells ; 

 Hatschek has pointed out the early exhibition of the bilaterality 

 in the ova of Teredo and Pedicellina ; Brooks, in the ova of the 

 American oyster. E. van Beneden, in various papers on Ascaris, 

 Tunicates, etc., has called particular attention to the importance 

 of the study of the promorphological relation of the ovum. 



