274 CEPHALOPODA. 



the whole embryo (Fig. 116 J3, ar, p. 255) and contains, here as there, 

 the rudiments of the arms. Those of the two ventral pairs are to 

 be seen first, becoming differentiated from the circular swelling as 

 rounded prominences. These are followed very soon by a third pair. 

 When these three pairs have become distinct, the swelling containing 

 them is prolonged laterally as a narrow process. As this becomes 

 more massive later, the fourth pair of arms develops from it, this 

 being finally followed by the fifth which lies the most dorsally (Fig. 

 128 A, a-ia.z). The order in which the arms arise is therefore the 

 same in Sepia as in Loligo vulgar is (p. 261).* Here also the pre- 

 hensile arm, as a rudiment, does not seem to be characterised in 

 any special way. 



Another characteristic of the germ-disc at this early stage (Fig. 

 128 A) is a pair of rather long, arched folds near the mouth, 

 the later development of which leads us to recognise them as the 

 posterior funnel-folds. Between these and the mantle lie two 

 comma-shaped prominences, the gills. The anterior funnel-folds also 

 (ytf) are already present as rudiments. In their neighbourhood, lie 

 the circular otocysts (Fig. 128 A, ot). These are the organs which 

 KOLLIKEB, as VIALLETON pointed out, regarded as the nuchal 

 cartilages. 



We have here, as in former descriptions of the first rudiments of external 

 organs, repeatedly had to speak of folds (e.g., the funnel-folds), but this 

 designation is not altogether justifiable, since, as VIALLETON has shown in 

 sections of young Sepia embryos, these and other prominences, are, for the 

 present, mere thickenings of the superficial cell-layer accompanied by slight 

 bulging of the same, but these thickenings soon rise into actual folds. This 

 is especially the case in the further development of the mantle and the funnel 

 in the next stages. 



While the organs mentioned have appeared as rudiments, the 

 germ-disc only at first extends slightly over the. yolk, occupying 

 merely a small part of it. The blastoderm also does not nearly cover 

 the yolk, but appears in the form of a broad ring beneath the germ- 

 disc. Over the rest of the egg, the yolk lies freely at the surface. 

 The germ-disc and the blastoderm are covered with cilia which are 

 also present in later stages, when the embryo has risen up from the 

 yolk (Fig. 129 C). 



* Our statements as to the rise of the arms rest upon our own observations. 

 Since the observations of KOLLIKER and VIALLETON agree with ours, there 

 would be no need to mention this, especially as only a few Sepia embryos at 

 this stage were available, had not the order of development of the arms been 

 less clearly or otherwise described in other cases. 



