in the ova of Echinus hrevispinosus (esoulentus) 9 and then 

 in Trieste on the smaller, more transparent ova of Echinus 

 lividus (saxatilis) , with the first, his attention was 

 attracted to the fact that inside the ovum, soon after 

 fertilization but before the beginning of division, a light 

 radiance appeared but shortly afterwards disappeared. Apply- 

 ing slight pressure upon the ovum, Baer could get an extended 

 vesicle (nucleus) or two vesicles situated side by side. 

 Noting the direction of the longitudinal axis of this extended 

 light region, Baer observed that after the yolk (ovum) divided 

 into two halves, the centers of these halves were situated in 

 the same axis. From these first observations he concluded 

 that "the processes in the internal region of the ovum precede 

 the division of the yolk and predicate it," The study of the 

 subsequent stages of division led Baer to the conclusion that 

 the externally observable phenomenon of division always 

 precedes the division of the transparent nucleus present inside. 

 The details of the ovum structure, using intravital observations 

 with little magnification, Baer of course could not investigate, 

 and discussions preserved the terminological confusion of that 

 time, when the relations between the nucleus of the immature 

 ovum (embryonic vesicle, or Purkinje's vesicle), the ovum 

 nucleus, and its nucleolus (Wagner's spot) were still not 

 established. 14 



For the first stage of development of the immature ovum, 

 Baer wrote, 



I consider the nucleus to be identical with that 

 part which is usually called Wagner's spot . , . . 

 Much later the part which is apparently considered 

 an embryonic vesicle occupies a great part of the 

 ovum. Because the small body shows, under pressure, 

 a great resistance, the name "spot" is not so 

 suitable. It seems to me very probable that the 



14. It must not be forgotten that even thirty years later 

 O. Hertwig thought that during the maturation of the 

 ovum the embryonic vesicle disappears, and the embryonic 

 spot becomes the nucleus of the mature ovum (O. Hertwig, 

 "Beitrage zur Kenntniss der Bildung, Befruchtung und 

 Teilung des tierischen Eies," MORPH. JAHRB., I (1875) } 

 III (1877) t IV, (1878) . 



468 



