Art. XVIII. — Notes from the Biological Lahoratory of the 

 Melbourne University. 



(With Plate VI.) 

 [Read December 12, 1889.] 



(1) On the Occurrence of a Partially Double Chick Embryo. 

 By A. H. S. Lucas, MA., B.Sc. 



Formerly, abnormal growths of plants and animals excited 

 interest as being curious marvels, monstrosities, so-called 

 liisvbs naturce. Now-a-days, tbey are studied in the hope of 

 discovering in them extraordinary, and perhaps therefore 

 especially instructive, manifestations of ordinary laws of 

 growth. 



The younger such abnormal forms are, the more light are 

 they likely to throw uj^on the difficult problems, latterly so 

 much discussed, concerning the physical causes of heredity 

 and variation. 



The chick will probably, owing to the ease with which 

 material for research can be procured and manipulated, 

 always furnish the standard type of development amongst 

 the higher vertebrates. It seems well then to record 

 occurrences of healthy chick embryos which depart in a 

 marked manner from the ordinar}^ lines of growth, especially 

 as such embryos appear to be somewhat rarely met with. 



The particular embryo under consideration had been 

 incubated about twenty-seven hours, and presents most of 

 the features characteristic of this time. The blood-vessels are 

 definitely forming in the vascular area. The vitelline folds 

 are clearly indicated. The central nervous system, and the 

 mesoblastic somites, have their usual appearance at this 

 stage. 



The embryo is, however, partially double. In front, it 

 forks to form two very distinct heads, of which one is larger 



