Nos. 455-456.] 



FASCIOLARIA. 



877 



futile attempt at segmentation where normally we should find 

 mitosis, but in this case the cell having the impulse to divide 

 but being powerless to do so by mitosis falls back on the easier 

 mode and does so by amitosis. We may call it cell senescence. 



The gastrulas before they have swallowed the food-ova are 

 (according to information and drawings access to which I owe to 

 the kindness of Prof. MacMurrich) very queer looking objects 

 on account of the very ample folds of the ectoderm to allow for 

 the distension which is to follow. One would expect that the 

 endoderm would be equally so, in order to receive the ova into 

 an endoderm-lined cavity, but after much study of this point I 

 am convinced that unusual as it is there is not enough endoderm 

 to enclose these ova, but only a very small amount reaching out 

 a short distance from the throat in all directions, as in Fig. 2. 



A^study of various series agrees in showing only one very thin 

 layer the ectoderm in contact with the ova except near the 

 stomod^eum. As development progresses the endoderm pushes 

 out around the food-ova, at first with very thin flat cells, which 

 later still become cubical and finally differentiate into the large 

 and vacuolated definitive cells of the liver, or the columnar cells 



