haeckel's recent additions to gastr/ea-theory. 59 



the Coelenterata, or Zoophytes, there are some excellent 

 examples of the type ; for instance, among the Ctenophora 

 and Siphonophora, as well as in Corals and Sponges. 



In Plate X two examples of the amphiblastic mode of 

 development as far as the Gastrula stage are given; the upper 

 series relates to the Annelid Fabricia, one of the Sabellidse, 

 and the lower to a Gasteropod, probably a Trochus. They 

 were both studied by Haeckel at Ajaccio. He recommends the 

 use of carmine and hiematoxylin as staining agents, the em- 

 bryo being rendered transparent by glycerine. The reader is 

 referred to the explanation of the plates for further details. 



The amphiblastic type of development presents a great 

 range of variation in respect of the amount of food-material 

 incorporated with that portion of the Monerula which is to 

 give rise to the endoderm-cells, and an accompanying varia- 

 tion in the number of cells produced by its cleavage and the 

 rate at which it cleaves. In this[way heterochronies are brought 

 about, the unencumbered ectoderm proceeding more rapidly 

 on its palingenetic development than does the endoderm. 



All amphiblastic Gastrulse are not epibolic, that is to say, 

 brought about by the growth or circumcrescence by the 

 ectodermal cells over the larger entodermal cells. In some 

 (for instance, Unio, Figs. 26, 27, 28) the relative growth is 

 such as to have the same appearance of embole (or entobol^), 

 which is observed in the Archiblastic series. As I pointed 

 out (loc. cit.) there is no sharp line to be drawn between 

 embolic and epibolic developments, but they are connected 

 by transition forms. Haeckel endorses this view, and further, 

 draws attention to the variation, which I have also insisted 

 on, in the share which the primitive endoderm cells take in 

 the production of the permanent alimentary canal. It is 

 impossible to say, with the observations at present before 

 us, to what extent these variations are exhibited in the 

 various groups of animals. In some cases endoderm-cells 

 are absorbed in the process of development, never taking 

 part in the formation of the wall of the primitive enteron 

 (archenteron). Such cells may be inclosed within the enteron 

 or lie outside its walls. Further, it appears (e.^. in Limnaeus 

 and Pisidium) that the archenteron is sometimes not directly 

 converted into the alimentary canal of the adult, but that 

 large portions of it are absorbed, or, as in Echinoderms, 

 Sagitta, Terebratula, &c.,jgive rise to a body-cavity. Haeckel 

 distinguishes the permanent enteron as metagaster. We may 

 call it the ' metenteron.' 



The problems here pressing for solution are of the greatest 

 consequence. The whole question of the nature of the coelom 



