GENEOLOGICAL. 373 



portant in the process, but against this we have the 

 facts as yet uncontroverted that a non-nucleated 

 ovum or even fragment of an ovum may be fertilised 

 and may develop to the larval stage (Boveri and De- 

 lage), and that artificial conditions may induce an 

 ovum to develop without a spermatozoon. Thus, 

 Loeb induced artificial parthenogenesis in sea-urchin 

 ova by placing them for a couple of hours in sea- 

 water, to which some magnesium chloride had been 

 added, disturbing the normal proportions of the 

 ions. There are also incipient experiments (Pieri, 

 Winkler, and others) on the effect of an extract of 

 sperm in stimulating the cleavage of the ovum. 

 Everything points to the desirability of extreme cau- 

 tion, but it seems likely that we have to distinguish 

 in fertilisation two distinct results (a) a mingling 

 of heritable qualities, and (&) a physiological stimu- 

 lus to division.* 



Since the formulation of the Cell-Theory, the de- 

 velopment of Embryology has been rapid, and this 

 may in part account for the insecurity of its general- 

 isations. We propose to refer to a few of these. 



Germ-Layers. The fertilised animal ovum di- 

 vides into a mass of cells a solid ball, or morula; 

 a hollow ball, or blastula; a convex disc on the top 

 of the yolk, and so on. The next great step is the 

 differentiation of two germinal layers the diplo- 

 blastic state. Of these the outer layer is called the 

 ectoderm or epiblast, and the inner the endoderm or 

 hypoblast. When the egg is not encumbered with 

 much yolk this two-layered stage most frequently 

 assumes the form of a thimble-shaped or barrel- 

 shaped embryo, whose cavity is the primitive gut or 



* See Geddes and Thomson. The Evolution of Sex, revised 

 (4th) edition, 1901. 



