THE GASTRAEA-THEORY, ETC. 157 



actual construction and form as the earlier condition of indi- 

 vidual development in representatives of all the animal 

 groups (except only the Protozoa), is a biogenetic fact of the 

 greatest significance, and confirms the safe conclusion accord- 

 ing to a biogenetic fundamental law, that all the phylse of 

 the animal kingdom (except the Protozoa) branch off from a 

 single common unknown root-form, which was formed 

 essentially like the gastrula. In the * Philosophy of the 

 Calcareous Sponges' (1. c, pp. 345, 347, 467) I have called 

 this primitive, long extinct root-formj which must have 

 existed already in the earlier primordial time (during the 

 Laurentian period) the Gastraea. The acceptation of this 

 root-form, whose next descendants during that period pro- 

 bably appeared in many different genera and species of 

 Gastraeada, is firmly established by the homology or mor- 

 phological identity of the gastrula in the most different 

 groups of animals. It is therefore a witness of special sig- 

 nificance that the cells of both germ-lamellse have everywhere 

 retained their distinctive characters (by inheritance). The 

 cells of the inner germ-lamella or entoderm are everywhere 

 distinguished by their undifferentiated condition ; their shape 

 is globular or irregularly polyhedric, their protoplasm is 

 opaque, granular, diffluent, with oil-globules, and is quickly 

 and intensely coloured by carmine ; their nucleus is generally 

 globular ; for the most part they are not vibratile. On the 

 other hand, the cells of the outer germ-lamella, or exoderm, 

 are further differentiated ; their form is mostly cylindrical or 

 conical; their protoplasm is pale, clear compact, with no oil- 

 globules, and is coloured more slowly and less intensely by 

 carmine ; their nucleus is generally elongated ; the exoderm 

 cells mostly vibratile.' These are apparently more strongly 

 modified by adaptation to the surrounding outer world than 

 the interiorly placed entoderm cells, which have more truly 

 preserved the original character of the morula cells. The 

 ontogenetic formation and increase also proceeds more rapidly 

 in the exoderm than in the entoderm cells. 



From the gastrula homologous in all the groups of 

 animals (except the Protozoa) follows necessarily the true 

 homology of the primitive rudiment of the intestine in all 

 animals, as well as the homology of the two primary germ- 

 lamellse, in all those higher animals which have lost the 

 original gastrula-condition by the latv of contracted inherit- 



' The differences between the protoplasm of the exoderm and entoderm 

 cells are altogether analogous to the differences between the hyaline cortical 

 layer (exoplasm) and the granular medullary layer (endoplasm) in the uni- 

 cellular animal bodies of the Infusoria, Amoeba, &c. 



\OL. XIV. NEW SER. h 



