Primitive Cell-layers, of the Enibryo. 



325 



its ap])lication to the five higher groups of animals, continually 

 receiving new,support from observation, and seems likely to 

 lead into most productive lines of research. 



The early changes in the developing spheroid of protoplasm 

 leading to the formation of organs may be summarily stated 

 as follows, so as briefly to put in view the fundamental cha- 

 racteristics which they present in different groups of the animal 

 series. 



Fig. 1. 



Fi^. 2. 



Fi-. 3. 



Cytode. 



Cell. 



Polyplast without central 

 cavity. (Optical section.) 



A. The reproductive spheroid is a non-nucleated particle of 

 protoplasm [Cytod^ Hck.), which either acquires a nucleus and 

 becomes a true cell (Hck.), or remains in the non-nucleated 

 condition ; this latter condition characterizes the Monera or 

 Protozoa hoinogenea, whilst the former is what is observed in 

 all the other groups commonly classed as Protozoa (from which, 

 however, the S])ongida are excluded, since they appear in tlie 

 next section). By differentiation of the primitive substance of 

 the plastid (cell or cytode), without fission of the original mass, 

 a cuticle and cuticular appendages, muscular fibrous layers, 

 cilia, contractile cavities, and, by the segmentation of the 

 nucleus, a reproductive germ- or sperm -mass may be former. 

 Division of the primary spheroid, wlien it does take place, 

 gives rise to new and separate individual spheroids, or to a 

 loosely aggregated colony of such spheroids, to be termed a 

 polyplast. In this polyplast there is no arrangement of the 

 units into definite layers.^ 



The organisms whose mode of growth is thus described may 

 be distinguished as Homoblastica. 



Notes to A. — The stock of the Homoblastica thus coincides 

 with the Protozoa with the exclusion of the Sponges, and con- 

 tains the following chief groups, the genetic affinities of 

 which must be hereafter discussed : — 1. llomogenea (embracing 



