III. 7 OSMOTIC PRESSURE 125 



centring in the nucleus first appears ; this then fades away, 

 and a definite aster with a centrosome is formed just to one 

 side of the nucleus; this divides to form the first amphiaster 

 (cleavage-spindle). Asters also arise independently of the 

 nucleus in the cytoplasm (cytasters) ; these contain centrosomes, 

 and may divide, and the cytoplasm divide round them. The 

 part played by the cytasters in development is, however, in- 

 significant; their activity soon comes to an end. The number 

 of chromosomes is one-half the normal number. This latter 

 statement is confirmed by Morgan, but denied by Delage, who 

 asserts that, as in egg fragments enucleated and subsequently 

 fertilized, the half number becomes doubled. 



LITERATURE 



E. BATAILLON. La pression osmotique et les grands probleines de la 

 Biologie, Arch. Ent. Mech. xi, 1901. 



E. BATAILLON. Etudes experimentales sur 1'evolution des Am- 

 phibiens, Arch. Ent. Mech. xii, 1901. 



C. B. DAVENPORT. The role of water in growth, Proc. Boston Soc. 

 Nat. Hist, xxviii, 1897-8. 



C. FERE. Note sur 1'influence de la deshydratation sur le developpe- 

 ment de 1'embryon de poulet, C. B. Soc. Biol. (10) i, 1894. 



A. GURWITSCH. Ueber die formative Wirkung des veranderten chemi- 

 schen Mediums auf die embryonale Entwicklung, Arch. Ent. Mech. iii, 1896. 



0. HERTWIG. Die Entwicklung des Froscheis unter dem Einfluss 

 schwacherer und starkerer Kochsalzlosungen, Arch. mikr. Anat. xliv, 1895. 



0. HERTWIG. Die experimentelle Erzeugung thierischer Missbild- 

 ungen, Festschr. Gegeribaur, Leipzig, 1896. 



J. W. JENKINSON. On the effect of certain solutions upon the develop- 

 ment of the Frog's egg, Arch. Ent. Mech. xxi, 1906. 



J. LOEB. Investigations in physiological morphology, Jouni. Morph. vii, 

 1892. 



J. LOEB. Ueber die relative Empfindlichkeit von Fischembryonen 

 gegen Sauerstoffmangel und Wasserentziehung in verschiedenen Ent- 

 wicklungsstadien, Pftuger's Arch. Iv, 1894. 



J. LOEB. Ueber eine einfache Methode zwei oder mehr zusammen- 

 gewachsener Embryonen aus einem Ei hervorzubringen, P/Higer's Arch. Iv, 

 1894. 



LITERATURE ON ARTIFICIAL PARTHENOGENESIS 



E. BATAILLON. Nouveaux essais de parthenogenese experimentale 

 chez les Vertebres inferieurs, Arch. Ent. Mech. xviii, 1904. 



G. BULLOT. Artificial parthenogenesis and regular segmentation in 

 an Annelid (Ophelia), Arch. Ent. Mech. xviii, 1904. 



Y. DELAGE. Etudes sur la merogonie, Arch. Zool. Exp. et Gen. (3), 

 vii, 1899. 



Y. DELAGE. Etudes experimentales sur la maturation cytoplasmique 



