EMBRYOLOGICAL INVESTIGATIONS 253 



eight, sixteen, or even thirty-two blastomeres, which 

 in different specimens were approximately equal or 

 markedly unequal in size. Sometimes a preliminary 

 change of form occurred without any further result, 

 the egg returning to its spherical shape, and pausing 

 for a time before recommencing the attempt to 

 segment. Segmentation sometimes commenced at 

 one pole, as in the telolecithal eggs of birds or 

 reptiles, with the formation of four or five small 

 segments, the rest of the egg breaking up later, 

 either simultaneously or progressively, into segments 

 about equal in size to those first formed ; while 

 lastly, in some instances segmentation was very 

 irregular, following no apparent law. Similar 

 modifications in the segmentation of the egg have 

 been described in the oyster by Brooks, in Anodonta 

 and in other Mollusca, and in Hydra. In the 

 different species of Peripatus there appear also 

 to be considerable variations in the details of 

 segmentation. 



It has long been known that the eggs of 

 Amphibia may vary greatly in the mode in which 

 the early stages of segmentation are effected ; and 

 recently Jordan and Eycleshymer have described 

 these variations in detail. The first cleft usually 

 corresponds with the median sagittal plane, dividing 

 the egg into two blastomeres, which give rise 

 respectively to the right and left halves of the 

 embryo. .The cleft may however be oblique, or 

 even transverse to this plane. The two first blasto- 

 meres are usually equal, but may be very unequal, 

 one being sometimes twice the size of the other. 



