1884] 91 ( [Hyatt. 



an upper layer of cells differentiated from the lower, an esoteric 

 as contrasted with an exoteric layer, the representative of these 

 being respectively the apicals and the basals in the earliest stages 

 of Calcispongiae, and in later stages the endoblast and ectoblast 

 cells. These last, however, may be somewhat mixed in deriva- 

 tion, since, as shown above, the endoblast may consist of esoteric 

 or apical cells, with a peripheral rim formed by cells of the exo- 

 teric or basal layer. 



In the next stage a temporary gastrula is often formed by the 

 invagination of the now fully established ectoblast. The aspect 

 of the embryo at this time in Schultze's figure (pi. 21, f. 24) is 

 precisely similar to my figure of the same stage in Halisarca, 

 except that this has an elongated embryo. The contraction of 

 the temporary blastopore in this embryo or in Halisarca might, 

 as we have suggested above, convert it into a planula with 

 ciliated e'xterior, and smooth membrane of amoeboid cells in 

 the interior, or a slighter contraction make it into a gastrula with 

 a narrow mouth, and every aspect of permanent invagination. 



We entirely agree with Metschnikoff that Sycandra is a spe- 

 cialized form, and that its embryology cannot be accepted as typ- 

 ical or primitive, as has been done by Schultze and Balfour. 

 Nevertheless the ontology has been very thoroughly worked out 

 by Schultze, and we think the development shows that it is a 

 form with concentrated development in which the gastrula 

 appears without the parenchymula, or with this and the cincto- 

 planula only obscurely indicated, the . first possibly by the solid 

 amphiblastula filled with cells derived from delamination of the 

 endoblast, and the latter by the iuvaginated gastrula and its blas- 

 topore band of larger ectoblast cells, which are the homologues 

 of the collar cells of other embryos. 



The thin-walled Ascones are the lowest of Porifera and 

 their embryology is necessarily the standard for phylogenetic 

 comparisons. The Physemaria of Haeckel may be in part, 

 or possibly wholly, true Protozoa, as shown by Saville Kent 

 and Carter (Mag. Nat. Hist., ser. 5, vol. i, 1878.). The former 

 figures an undoubted Haliphysema, one of Haeckel's types 

 of Physemaria, which has an attached, club-shaped body, cov- 

 ered with particles of foreign matter and spicules accumulated 

 upon and incorporated in the external sarcode. It has, also, long 



