AMONG SIMPLE SAECODE OK&ANISMS. 263 



origin from nucleated cells, whether eggs or spores ; and in these, 

 as insisted on by Haeckel, genuine cytodes cannot occur. Here 

 all the later plastids which compose them must have arisen from 

 the genuine cell and, like this, must have been originally nucleated. 

 If non-nucleated plastids, such as the red blood-corpuscles of 

 Mammalia, should show themselves, these must have been produced 

 by retrogradation — a loss of the nucleus in what was originally 

 a genuine nucleated cell. In order to distinguish them from 

 the true non-nucleated cytode, he gives them the designation of 

 " Dyscytodes." 



Among the forms which I am now about to bring before you, 

 many examples of both true cells and cytodes will have to be 

 adduced. 



Hertwig and Lesser* have described a number of low Jmoeba- 

 like organisms and of other rhizopodous forms of fresh water. 

 They have worked out with much care their structure and affi- 

 nities, and have attempted a general exposition of their organiza- 

 tion and systematic position. 



They all consist of masses of protoplasm in which a nucleus 

 with nucleolus are almost always developed, and which, besides 

 these, include a greater or smaller number of vacuoles, which may 

 be either contractile or non-contractile. In the more purely 

 amoeboid forms their bodies have no definite shape, and are, for 

 the most part, absolutely naked ; but they are occasionally enve- 

 loped for a greater or less extent in a thin pellicle, which is ex- 

 creted from the surface of the protoplasm, and follows all its 

 changes of form, while, in rare cases, they are covered by foreign 

 particles agglutinated together. In others, however (Monothala- 

 mia, Hert. & Less., and Heliozoci), there exist more definite protec- 

 tive structures either in the form of external hard shells or of firm 

 membranous cases, or of variously arranged spines and spicules. 



In morphological value none of them pass beyond the stage of 

 a simple cell, or, at most, of two or more cells fused together into 

 a single protoplasm-mass without any tendency to the formation 

 of tissues or the differentiation of organs. In the protoplasm, 

 however, may frequently be distinguished two layers or zones — an 

 external layer (ectosarc), clearer and more homogeneous, and an 

 internal layer (endosarc), less transparent and more loaded with 

 granules. These two layers, for the most part, pass gradually 

 into one another. 



* " Ueber Rbizopoden und deuselben naheBtebei;de Organismen," Arch, f, 

 miki-. Anat. vol. x. Suppl. 187^1. 



23* 



