62 INVERTEBRATE MORPHOLOGY. 



" winter eggs " which develop after fertilization. Here no 

 difference of form exists betwe.en the two generations, but 

 such cases, as well as those in which two sexual generations 

 unlike in form and habitat alternate with each other, are 

 usually associated with the more typical examples as in- 

 stances of heterogony. 



LITERATURE. 



0. Hertwig. Beitrage zur Kenntniss der Bildung, Befruchtung und Theilung 

 des thierischen Eies. Morpliolog. Jahrbuch, i., in. and iv. 1875-78. 



E. van Beneden and A. Neyt. Noiivelles recherches sur la fecondation et la 



division mitosique chez FAscaride megalocephale. Bulletin de la Societe 



royale Belgique, xiv. 1887. 



Th. Boveri. Zellenstudien. Jenaische Zeitschrift, xxn. 1888 and xxiv. 1890. 

 H. Fol. Le quadrille des centres. Archives des Sciences pliys. et naturelles 



Geneve, xxv. 1891. 

 0. Hertwig. Vergleich der Ei- und Samenbildung bei Nematoden. Arcliiv 



fiir mikrosk. Anatomic, xxxvi. 1890. 



F. M. Balfour. A Treatise on Comparative Embryology. London, 1880. 

 E. Metschnikoff Embryologische Studien an Mcdiiu n. Vienna, 1886. 



E. Haeckel. Die Gastrula und die Eifurchung der Thieve. Jenaische Zeit- 

 schr., ix. 1875. 



