Minot.] 166 [April 18, 



In almost every case the entodermic cells are larger and less nu- 

 merous than those of the ectoderm. This distinction is obviously 

 necessary on account of the mutual relations of the two primitive 

 layers. The ectoderm has to grow around the entoderm, which it 

 can do only by acquiring a greater superficial extension — this the 

 ectoderm does by dividing very quickly at first into small cells. After 

 the entoderm is fully enveloped it may then continue to grow until 

 its superficies is much greater than that of the outer layer, within 

 which, however, it still finds room by forming numerous folds, thus 

 gradually reaching the condition of the higher adult animals, where 

 the intestine sometimes has an enormous surface, but is nevertheless 

 contained within body walls presenting much less surface. It is 

 therefore only during the early stages of the segmentation of the 

 yolk that we find the hypoblast expanding more slowly than the 

 epiblast. 



The difference between the two rates of growth is very variable. 

 Among the Molluscs there collects a small amount of finely granular 

 matter at one pole of the egg, while the rest is filled with large and 

 numerous nutritive granules imbedded in a fine protoplasmic net- 

 work. It is well known, through the observations of Lacaze-Duthiers 

 on Dentalium, Flemming on Lamellibranchs, Fol on Pteropods and 

 Heteropods, Lereboullet, Loven, Strecker, Leydig, and E. Lank ester, 

 etc., on various other Molluscs, that this finely granular matter (Bil- 

 dungsdotter, formative substance) segments rapidly, forming the ecto- 

 derm and enclosing the yolk, which meanwhile divides into only a 

 few large cells, which vary greatly in size and number in different 

 species, but in all cases enter into the composition of the entoderm. 

 It is important to recollect that the yolk in the instances above cited, 

 is contained in cells belonging to the inner germinal layer. It is fur- 

 ther known that both primitive layers are continuous, forming a 

 vesicle (Blase) within which there is a 'cavity filled with cells that 

 ultimately make the mesoderm. In all cases where the process of 

 segmentation occurs in such a way as to construct a single walled 

 vesicle, one pole forms the ectoderm, the other the entoderm; when, 

 however; no yolk granules are developed the difference in the two 

 rates of growth or segmentation is much less marked. Thus in the 

 Echinoderms, during the planula stage, the cells that ultimately 

 compose the ectoderm divide more rapidly than the others — conse- 

 quently the cells destined to become the walls of the primitive diges- 

 tive cavity appear larger. The same distinction appears among 



