EMBRYONIC DEVELOPMENT 367 



followed. The aspiculous embryos are spherical and attain a maximum diameter of 

 0-36 mm. and show an inner mass of large irregular cells with a thin layer of flattened 

 cells on the outside. From this point a decrease in size takes place to 0-3 mm. diameter, 

 the central mass becoming more compact and the outer layer being clearly marked off 

 as a thick layer, 0-035 ^™- thick, of columnar cells. At first the outer layer is of uniform 

 thickness throughout. The first spicules to appear seem to be the chelae, which measure 

 0-026 mm. chord, as against 0-035 ii^"'^- i" the adult, but do not otherwise present any 

 peculiarities. They are scattered evenly through the central mass of cells. Small 

 acanthostyli, 0-07 mm. long and difl^ering considerably from those of the adult (cf. 

 Fig- 54 g), then appear, also scattered through the substance of the central mass. A re- 

 arrangement then takes place, the acanthostyli finally being found in a bundle at the 

 centre of the embryo and towards one pole, their apices centrifugally directed, with the 

 chelae in a diff'use corona around the inner end of the bundle. At the same time, the 

 outer layer of columnar cells becomes much thinner near the outwardly directed bases 

 of the acanthostyli (Fig. 54 /). This may, by analogy, be regarded as the aboral pole. 



The specimens recorded by me (1929, p. 435) from McMurdo Sound, taken in 

 February, contain large aspiculous embryos. 



Dictyociona discreta (Thiele) (Fig. 547). One specimen, collected in March, contains 

 aspiculous embryos only, but a second specimen, collected in April, has a few spherical 

 embryos, 0-32 mm. diameter, containing rare, almost indefinable, acanthostyli, and 

 numerous others 0-25 mm. diameter, with a layer of acanthostyli measuring 0-07 by 

 0-002 mm. The acanthostyli are all lying tangentially to and just beneath the surface of 

 the embryo. In a third specimen, collected in July, a single embryo was found con- 

 taining numerous acanthostyli, arranged in a dense bundle at the centre, and numerous 

 chelae of 0-008 mm. chord (cf. the oxea in Callyspongia flabellata, sp.n.. Fig. 54^). 



Eurypon miniaceum, Thiele. Collected in October, contains ova and very young 

 embryos. 



Hymedesmia irritans, Thiele. A single specimen, collected at Tristan da Cunha on 

 February i, contains spherical embryos, aspiculous, measuring o-i6 mm. in diameter. 



Hymeniacidon sanguinea (Grant). One specimen, collected on July 28 from Angola, 

 contains numerous embryos up to 0-2 mm. in diameter. Most of the embryos are 

 aspiculous, but some contain styli, with faintly irregular surfaces, measuring 0-12 by 

 0-003 mm. These spicules are at first arranged tangentially to the surface, but later 

 migrate inwards to lie in a confused mass towards the aboral (.?) pole with their apices 

 directed towards the centre of the embryo. 



Hymeniacidon diibia, sp.n., collected in March, contains ova and very young 

 embryos. 



Plicatellopsis flabellata, sp.n. The specimen, collected on June 22, contains a few oval 

 embryos, 0-07-0-1 mm. in longest diameter, in early stages of development. 



Stylocordyla borealis (Loven), subsp. acuata (Kirkpatrick). A single specimen, col- 

 lected on December 15, contains numerous embryos in an advanced stage of develop- 

 ment and a few that are entirely aspiculous. Embryos have been found in the Terra 



17-2 



