332 Mr. H. J. Carter on the 



former, as before stated, in Halisarca loLulari's, and shown in 

 the illustration of the ovum of Esperia cpgagropila, var. 

 (PI. XXI. fig. 25) — that is, probably immediately after tlie 

 subdivision of the yelk has been completed. 



Third Period. 



That from the formation of the embryo to its fixation or 

 stationary position. 



As, however, the spheroidal ovum elongates, the posterior 

 end becomes marked by the development of a brownish-yellow 

 coloured mass of cells, which subsequently take on a globular 

 form (PI. XXI. fig. 22, a), or are arranged round the base in 

 a circle (fig. 21, _/), contrasting strongly with the opaque 

 white colour of the embryo. The latter, now becoming fully 

 matured, bursts through its capsule and swims into the water. 

 At this time it is very large, as may be seen by the illustrations 

 (figs. 21 & 22), compared with the embryo of Halisarca lohu- 

 laris (PI. XX. figs. 11 & 12), and especially that of Grantia 

 compressa (figs. 13 & 15), which, for this purpose, are all 

 drawn, as before stated, to the same scale. In shape it is cy- 

 lindrical (fig. 21), conical at one end and round truncate at the 

 other, measuring l-30th inch long by l-90th inch wide. At 

 the anterior end is a papillary projection (fig. 21, e, & fig. 22,/), 

 and at the posterior one the brown cells mentioned (fig. 21,/ 

 and fig. 22, a) ; w^iile all the rest of the surface is covered by 

 the ciliated ectoderm or layer of minute monociliated cells 

 (fig. 21,4. 



(In the illustrations this ciliated layer has only been gene- 

 rally represented, by dots all over the body, in the embryo of 

 Halisarca lohularis [PI. XX. fig. 11], but for convenience has 

 been omitted in the rest, or rather reduced in representation to 

 the dotted line roimd the margins of the other figures respec- 

 tively.) 



Besides the cilia of the surface there is a line of very large 

 ones of a distinct kind, and five times as long as those of the 

 ectodermal layer, which encircles the base and forms a mutual 

 boundary to the brown-coloured mass of cells on one side and 

 the ectodermal ciliated surface on the other (fig. 21, ^, and 

 fig. 22, e). 



The cells of the brown-coloured mass, on the other hand, 

 which are much larger than those of the ectodermal layer, 

 present themselves, as before stated, in a globular form (fig. 

 22, a) or in a simple circle round the base (fig. 21,/). 



In progression the embryo swims with the papillary or 

 pointed end foremost, and rotates upon its long axis from left 



