lii THE VOYAGE OF H.M.S. CHALLENGER. 



It thus appears that there is one developmental stage fairly constant in the 

 Halichondrina, in which the embryo consists of a sac of small, probably prismatic, 

 ciliated ectodermal cells, enclosing a central mass of mesodermal (?) tissue containing the 

 developing spicules. In some cases, perhaps in all (for it would be difficult owing to the 

 nature of the case to make sure of the contrary) the ectoderm appears to be absent from 

 one pole, at which the mesoderm (?) comes to the surface. This stage probably nearly 

 corresponds with one figured by Keller for his Chalinula fertilis,^ by Carter for 

 Halicliondria simulam and Esperia wgagropila,^ by Marshall for Reniera Jiligrana,^ 

 and by Schulze for Euspongia officinalis adriatica.* 



There is one other type of embryo, observed by us in Esperella lapidiformis only, 

 which seems worth noticing in this place. The embryos (PI. XVI. fig. 2a, e) are very 

 numerous, more or less spherical, measuring about 0'5 mm. in diameter. Each is 

 enclosed in a membranous capsule, and is composed mainly of a dense, solid, finely 

 granular mass of tissue (PL XLVIII. fig. 1, a), with small nucleoid bodies scattered through 

 it. This structure prevails throughout the embryo excepting at one pole, where there is a 

 hemispherical cap of large cells (PI. XLVIII. fig. 1, c), appearing in sections to be polygonal 

 from mutual pressure, each with a large oval nucleus (?i), about 0"01 mm. in longer diameter, 

 and a nucleolus. The cap of cells appears sometimes to be slightly shrunk away by the 

 action of the spirit from the remainder of the embryo. This would seem to be a much 

 earlier stage of development than those above described ; there are as yet no traces of 

 spicules, 



1 Zeiischr.f. iciss. Zool., Bil. xxx., pi. xix. fig. 15. 



2 Ann. and Mag. Nat. Hist., ser. 4, vol. xiv., pi. xxi. figs. 21, 22, 25. 

 ' Zeitschr. f. wiss. Zool, Bd. xxxvii., pi. xiv. figs, b, c. 



^ Zeitschr. f. wiss. Zool., Btl. xxxii., pi. xxxviii. fig. 4. 



