1877.] 177 [McCrady. 



of protoplasm in Rhizopods, the latter of which are probably capable 

 of serving a digestive function, it is easy to conceive that in this 

 state similar unions of the radial extensions of the entoprotembryo 

 may finally traverse the yolk mass in every direction, and even unite 

 in a thin uniform layer of protoplasm all over the periphery of the 

 egg. In that case the entoprotembryo is converted into a pantoprot- 

 embryo, which indeed would naturally suggest itself as the type of 

 which the other two forms in all cases are specializations. 



17. In this case the process of segmentation is explained accord- 

 ing to our theory as identical with the preceding, except that every 

 segmentation is accompanied, as in Infusoria, by an elongation and 

 division of the nucleus, or central massing of the protoplasm of the 

 protembryo. If there exist any case in which segmentation occurs 

 before the entoprotembryo has been converted into the pantoprotem- 

 bryo (and such a case is quite conceivable), the energy of segmenta- 

 tion must of course there operate exclusively in a centrifugal manner. 



18. The essence of the theory, therefore, consists in this — that 

 the protembryo is conceived in all cases as an amoeboid body of 

 protoplasm which is prepotent over the protoplasm of the yolk, and 

 under all circumstances of position in reference to the yolk, appropri- 

 ates it as an Amoeba appropriates its food; the process of segmenta- 

 tion being a modified form of such appropriation, by which the whole 

 yolk mass is early divided into a great number of small particles, 

 each contained and enveloped by a distinct cell-like portion (splicer a 

 segmentula) of the protembryo. These cell-like portions of the pro- 

 tembryo are really, in the wide sense of the word, true cells, and 

 subsequently by differentiation and multiplication become the cells of 

 the mature organism, 



19. The segment spheres (sphcerce segmentulce) of the embryo, or 

 blastoderm, are held together by some unknown agency, while ap- 

 pearing to be mechanically unconnected with each other. Moreover, 

 when thus separated for a time (as when first formed), they approach 

 each other again and then remain in contact, or are even pressed 

 together with so much force as actually to be flattened one against the 

 other, so that eventually after one or many subdivisions, they are 

 converted by mutual compression from round to polygonal forms. In 

 this condition they are variously disposed in different animals. In 

 some they constitute a solid aggregation of spheres, in others a hol- 

 low spheroid whose cavity is known as that of von JBaer. This cavity 

 is usually regarded as empty, or at least containing no part of the 



