THE DEVELOPMENT OF SPONGES. 567 



The Opalinina must clearly be arranged among the Infu- 

 83ri((. Stein regards them as simply the lowest forms of the 

 Holotricha, but it will probably be safer to consider them as 

 a distinct group, standing in somewhat the same relation to 

 the Ciliata as the 6rm/"/v///'A# do to the Amoebae. 



SECTION II. The elucidation of the problem of the mode 

 of development of the Sponges has been greatly advanced 

 by the investigations of Oscar Schmidt, 1 Schulze, 3 and espe- 

 cially of Barrois, 8 which confirm the assertion of Metschnikoff 

 that" the vesicular morula which constitutes the early condi- 

 ti'Hi of the sponge-embryo consists of blastomeres of two 

 kinds; those of the one-half of the spheroidal or flattened 

 embryo being elongated and flagellate ; those of the other, 

 rounded, granular, and nonciliated. Schulze and Barrois 

 have independently ascertained that the latter region some- 

 times undergoes partial invagination ; and that a cup-shaped 

 body is produced, composed of an epiblast formed of flagel- 

 late cells and a hypoblast of spheroidal, non-ciliated cells. 

 Thus the " gastrula " stage of Haeckel may exist, though it 

 is not formed by dela r nination, as he supposed, but by invagi- 

 nation. But it appears that this gastrula-stage does not 

 always occur, and that whan it does, it is transitory, in so far 

 ai the hypoblastic cells subsequently enlarge, protrude be- 

 yond the epiblastic cells, and give rise to the free ovate em- 

 bryo formed of a ciliated and nonciliated half, which has so 

 often been observed. According to Barrois's observations, 

 this free swimming larva fixes itself by its nonciliated hypo- 

 blastic h ilf, and tha hypoblastic cells are invested by those of 

 the epiblast, which thus constitute the whole outer covering 

 of the young sponga. The central cavity of the sponiro, 

 which represents the archenteron, arises in the midst of the 

 included hypoblastic cells, while the osculum is a secondary 

 opening, formed apparently by an invagination of the ecto- 

 derm, and has nothing to do with the primitive blastopore. 

 Thus even the simplest sponge has passed beyond the gas- 

 trula-stage. 



Sshulze has made the important discovery that, in Si/- 



" Zur Orientirung uber die Entwickelung der Spongier! " (Zritsshriftfitr 

 wiss. Zoh>i'H\ 1875^; and " Nochmals die Gastrula der Kalkschwamme " 

 (" Archiv i'. Mikr. Anat.," 1876). 



aU Ueber den Bau und die Entwickelunsr von Si/cnndra rapAanut" (Ztit- 

 gchrift fnr wiss. Zoolngie, 1875) ; and tl Zur Entwickelungsgeschichte von Sy- 

 candra" (ibid., 1876). 



" Annales des Sciences Naturelles," 1876. 



