CLEAVAGE AND DIFFERENTIATION 



95 



yolk is left near the surface, and this interferes with the process of 

 gastrulation. The lip of the blastopore becomes as it were split on 

 this obstacle, and invagination takes place in opposite directions, 

 away from the streak of yolk. In other cases, the blastocoel is dis- 

 placed, and it seems that the pressure within it causes the cells which 

 form its wall to present an obstacle on which the blastopore lip 



'X^/i 



^'1as;«g(»^-i 



VI^ 



Fig. 42 



Double monsters of duplicitas critciata type, produced by inverting the 2-cell 

 stage of frogs' eggs. (After Schleip and Penners, from Morgan, Expemnental 



Embryology , Columbia University Press, 1927, fig. 157, p. 393.) 



becomes split, and likewise forks. Each portion of the blastopore lip 

 then invaginates on its own, and gives rise to the essential features 

 of an embryo, in so far as this is mechanically possible^ (fig. 42). 

 Double monsters have also been obtained in the frog simply by 

 fertilising over-ripe eggs. The cleavage of such eggs is abnormal in 

 that the blastomeres of the vegetative hemisphere are relatively 

 much too large. Presumably, the physiological condition of such 



^ Penners and Schleip, 1928. 



