Sars,10 van Beneden,H and Quatrefages-^ and must be 

 mentioned. The first four investigated mollusc development, 

 and the last studied the development of annelids. They 

 presented some stages of ovum division, but did not broach 

 the subject of the internal processes occurring in it nor 

 of the fate of the spheres of division. 



A great part of the works of that time concerning the 

 development of invertebrates was illustrated by the study of 

 different types of reproduction, and also by the description 

 of the structure and the transformation of different larval 

 forms which sometimes did not yield to systematic determina- 

 tion and figured under different specific names (126) . The 

 investigations of forms of reproduction became especially 

 popular after Steenstrup showed the wide distribution of 

 the alternation of sexual and asexual generations of many 

 invertebrates; the application of this empirical regularity 

 to groups of animals not investigated before this time 

 constituted the majority of works in the present sphere. 

 The single base for generalization concerning early embryonic 

 development is thought to be the cellular theory formed 

 shortly before this. Using extremely imperfect microscopic 

 techniques, the embryologists of the first half of the 

 nineteenth century posed the question, can the spheres of 

 division be called cells, and are the cells of which the 

 embryo consists the direct descendants of the primary 

 spheres of division? They attempted to trace the fate of 

 those existing in the unfertilized ova "embryonic vesicle" 

 (and "embryonic speck"), i.e. to explain whether these 



10. M. Sars, "Beitrag zur Entwickelungsgeschichte der 

 Mollusken und Zoophy ten," ARCH. NATURG . , 6 (1840), 

 pp. 196 - 219. 



11. P. J. van Beneden et Ch. Windischmann, "Recherches sur 

 l'embryogenie des Limaces," ARCH. ANAT . PHYSIOL. 

 (1841) , pp. 176 - 195. 



12. A. de Quatrefages, "Sur l'embryogenie des Annelides," 

 ANN. SC. NAT., 3 Ser. Zool. 8 (1847), pp. 99 - 102. 



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