Hyatt.] 94 [March 5, 



mation of the granular portions of the mesoderm may begin to 

 appear by delamination at a very early stage in Halichondria. 

 It will be noticed, also, that in the parenchymula and gastrula, 

 as in the adult, the three membranes are similarly characterized. 

 The external and internal are similar and membranous, while the 

 middle layer is remarkably loose in construction and of a more 

 primitive structure, a mere aggregate of cells. 



According to Schultze's observation on later stages in Sycan- 

 dra raphanus, the ciliated and collared cells of the endoblast fin- 

 ally become invaginated, forming a permanent gastrula and a 

 true blastopore ; and this gastrula is really an ampulla of primi- 

 tive form. The larva becomes fastened by the blastoporic end, 

 closes the pore, and opens another, the primitive cloaca, at the 

 free end. Metschnikoff (Zeitschr. Wissen. Zoo]., vol. xxxn, p. 368) 

 states that he had always held the opinion that the flagellated 

 membrane was invaginated to form a gastrula in Sycandra ; and 

 at the time Schultze published was ready to prove his view with 

 sections and preparations. His figures 7-8 (pi. 21) seem to be 

 the ones referred to, and his figure 15 (pi. 23) and description 

 of the three-layered embryo of Ascetta primordialis leave no 

 doubt that ga stimulation occurs in the ciliated endoblast among 

 Ascones, as well as in Sycones. 



These investigations show that the permanent gastrula occurs 

 by the invagination of the endoblastic membrane in all forms, 

 and that this membrane before invagination takes place is com- 

 posed of cylindrical, flagellated cells similar to those which 

 appear subsequently in the ampullae. The transient gastrula 

 arising from invagination of the ectoblast in Sycandra is, as 

 Schultze stated, not followed by any structural result, and is dis- 

 tinct from the similar transient gastrula of Carneospongia. These 

 last, being formed by the endoblast, may be legitimately supposed 

 to have some connection with the final appearance and genesis ot 

 the permanent gastrula, which is present in all forms. Confu- 

 sion has been occasioned by this primitive and transient occur- 

 rence of gastrulation. If our views are correct, however, the 

 transient gastrula among Carneospongiae may be always distin- 

 guished by the fact that it is found at stages when the layers are 

 easily distinguishable from each other as ciliated ectoblastic and 

 endoblastic amoeboidal cells; whereas the permanBnt gastrula 



