haeckel's recent additions to gastr^a-theory. 57 



connecting the Archiblastic with the Amphiblastic and with 

 the Periblastic, and the Amphiblastic with the Discoblastic 

 and Periblastic. To the student of Embryology it will be 

 clear enough in a general way that these terms are meant 

 to imply, 1st, Archiblastic, the primitive egg-segmentation 

 into equi-formal cleavage spheres, and the subsequent assump- 

 tion of the simple two-cell-layered gastula-form with or with- T" 

 out apical orifice, either by invaginatton or delamination ; \ 

 2nd. Amphiblastic, the egg-segmentation into inequi-formal 

 cleavage-spheres, one portion of which is charged with the 

 matrificial food-material, and becomes overgrown by the 

 smaller cells free from food-material (epibole, Selenka). 

 3rd. Discoblastic, the segregation of a small disc or klastic 

 patch from the rest of the egg, in which disc alone the 

 cleavage process is carried on. 4th. Periblastic^ the segre- 

 gation not of a disc, but of a complete superficial layer of 

 klastic material around the inert food-material. 



The Archiblastic Type. — Examples of this are met with 

 among the Coelenterata in Sponges, Hydroids, Medusae, and 

 Corals ; among Worms, as in the case of Sagitta and Phoronis ; 

 among Molluscs, as in the Brachiopoda and some Lamel- 

 libranchs and Gastropods ; among Echinoderms generally ; 

 among certain low forms of Arthropoda, as for instance some 

 Branchiopoda and perhaps the Pteromalina (Tracheata) ; 

 among Vertebrates, in the case of Amphioxus and some 

 Ascidians.^ Haeckel gives the development of a very 

 simple form of polyp, as typical of the Archiblastic type. 

 This very curious little polyp, which he names Gastro- 

 physema, is similar to the Haliphysema of Bowerbank, and 

 appears even in its adult condition to get no further than 

 the Gastrula stage : it is a true Gastrsea. It was originally 

 described by Carter as Squamulina scopula, that observer 

 having taken it for a Polythalamian. 



In the plates which we reproduce no figure is given of 

 the Archimonerula, Archicytula, Archimorula, which the 

 reader can easily picture to himself; but in fig. 20 we have 

 the Archiblastula of an Actinia, in fig. 21 its Archigastrula 

 formed by invagination; in fig. 29 the Archiblastula of 

 Limnseus is given, and in figs. SO and 31 its Archigastrula. 

 Fig. 17 gives the Archigastrula of a Calcareous Sponge 

 (Asculmis), fig. 22 that of a Medusa (Pelagia), fig. 23 that 

 of a worm, Sagitta, fig. 25 that of a Brachiopod (Argiope), 

 fig. 33 that of an Echinoderm (Asteracanthion). 



Haeckel holds (and this is an important point for theoretical 



* Professor Haeckel is not responsible for the classification here adopted 

 of the Tunicata under the great group of Vertebrj^ta, 



