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JORDAN. [Vol. VIII. 



still spherical the axis of the embryo is seen to lie in the long 

 axis of the capsule, and hence at right angles to the first 

 cleavage ^Dlane. There is usually variation, often amounting to 

 as much as 45° or 50°, but, in by far the great majority of the 

 eggs I have examined, the axis of the embryo is approximately 

 at right angles to the first plane of cleavage. 



In the Q.g% of the frog, as is well known from the researches 

 of Newport, Pfluger, Roux and others, the direction of the axis 

 of the embryo coincides with the first cleavage plane. Through- 

 out the animal kingdom there can be discovered no constancy 

 in the relation of first cleavage plane and embryonic axis. 

 There are several well-established instances where, as in the 

 frog's ^%g, the two planes coincide, and recently cases have 

 multiplied where the opposite is true and the planes are at 

 right angles (Nereis, Wilson ; Crepiditla, Conklin ; Jcsra, Mc- 

 Murrich). Miss Clapp ('9l) has shown that out of twenty-three 

 embryos of Batrachus, three showed coincidence of the axis of 

 the embryo with the first cleavage plane. " Fourteen of the 

 embryos had the head directed towards the right of the first 

 line of cleavage, the axis of the body being at an angle with the 

 first cleavage plane of from 30° to 70°. In the remaining six 

 the head was to the left of the first cleavage plane, the angle 

 varying as before." It is a plausible* hypothesis that when 

 planes and axes do coincide or cross at right angles it is because 

 both are determined by the same external conditions and not 

 necessarily because there is any causal nexus between the two. 

 It is possible that the same external factors that determine that 

 the first spindle shall lie in a certain axis determine also the 

 direction of the embryonic axis. 



The most recent work in experimental embryology seems to 

 confirm this inference and to demonstrate that great stress 

 cannot be laid upon the relations of early cleavage planes and 

 embryonic axes. When we know that four perfect gastrulae of 

 Amphioxus can be produced from the first four quadrants of the 

 Qgg, there is no basis for regarding the direction of the first 

 cleavage plane as a matter of supreme importance. The early 

 variations in the position of the nuclei in the newt (Plate 

 XVII), and the irregular torsions of the first cleavage planes 



