28 EMBRYOLOGY. 



6. Eggs may be divided into several groups and sub-groups ac- 

 cording to their development from cells of the ovary alone, or from 

 cells of the ovarium and vitellarium, MS well as according to the 

 distribution of the deutoplasm, as exhibited in the following 

 scheme : 



I. Simple eggs. (Development from cells of the ovary.) 



A. Eggs with little deutoplasm uniformly distributed through 



the egg (alecithal*). (Amphioxus, Mammals, Man.) 



B. Eggs with abundant and unequally distributed deutoplasm. 



(1) Eggs with polar differentiation (telolecithal), with deuto- 



plasm having a polar position, with animal and 

 vegetative poles. (Cyclostomes, Amphibia.) 



(2) Eggs with polar differentiation, which are distinguished 



from the preceding sub-group by the fact that with 

 them there has been effected a still sharper segregation 

 into formative yolk (germ-disc) and nutritive yolk 

 into a part which is active during development and a 

 part that is passive. (Eggs having polar differentia- 

 tion with a germ-disc. Fishes, Reptiles, Birds.) 



(3) Eggs having central differentiation with central deuto- 



plasm (centrolecithal) and superficially distributed 

 formative yolk (blastema, Keimhaut}. (Arthropods.) 

 II. Compound eggs. (Double origin from cells of the ovarium 

 and vitellarium.) 



LITERATURE. 



Baer, C. E. von. De ovi marnmaliurn et homiuis genesi epistola. Lipsiae 



1S27. 

 Beneden, Ed. van. Eecherches sur la composition et la signification de 



1'cBiif. Mem. cour. de 1'Acad. roy. Sci. de Belgique. T. XXXIV. 1870. 

 BischofF. Entwicklungsgeschichte des Kanincheneies. 1842. 

 Flemming. Zellsubstanz, Kern- mid Zelltheilung. Leipzig 1882. 

 Frommann, K. Das Ei. Kealencyclopadie der gesammten Heilkunde. 2. 



Auflage. 

 Gegenbaur, C. Ueber den Ban und die Entwicklung der Wirbelthiereier rnit 



partieller Dottertheilung. Archiv f. Anat. und Physiol. 1861. 

 Guldberg. Beitrag zur Kenntniss der Eierstockseier bei Echidna. Sitzungsb. 



d. Jena. Gesellsch. (1885), p. 113. 

 Hensen. Die Physiologic der Zeugung. Hermann's Handbuch der Physio- 



logie. Bd. VI. Theil II. Leipzig 1881. 



* The translator has been accustomed for several years to use the word 

 homolecithal instead of alecithal, heterolecithal being employed as a coordinate 

 term to embrace telolecithal and centrolecithal eggs. 



