537 



while at the other the cells are greatly larger and fewer in number. 

 The blastoderm thus formed produces a lighter spot on one side of the 

 egg which strikingly resembles the primitive cumulus of the Arachnids. 

 With the formation of this blastoderm the secretion of the Blastoderm- 

 haut (Amnion of Packard, deutovum of my former paper ^) begins. 



3) In from 8 to 1 1 days after impregnation (the period varies in 

 eggs of the same lot) a small circular pit appears in the centre of the 

 primitive cumulus. This I regard as the blastopore. This soon becomes 

 triangular and then elongates, while on the next day a second cloud 

 appears behind the first, but connected with it. At first the second 

 cloud is smaller but it soon attains equality with the primitive cumulus 

 and soon surpasses it. During this process the outlines become indis- 

 tinct, more so than in Balfour's 4 pi. XIX fig. 1 which in other res- 

 pects, except in length agrees well. During this process the blastopore 

 increases in length backwards, in the shape of a shallow groove (pri- 

 mitive groove) the enlarged anterior end of Avhich continues to mark 

 the original site of the first appearance of the structure. This primitive 

 groove runs back into the posterior cloud and fades out behind. A 

 second lighter area has now become prominent along the margins of 

 the blastopore and its posterior continuation, produced by the prolife- 

 ration, as shown by sections, of mesodermal cells from the margins. 

 These wander in between the rest of the blastoderm (ectoderm) and 

 the yolk (entoderm) cells which occupy the interior. Gastrulation pro- 

 duces no entoderm. 



4) In 15 days this primitive groove has become less distinct, 

 through the flattening of its walls ; while the germinal area, now out- 

 lined by the limits of the extension of the mesoderm, has become 

 divided by the appearance of a transverse groove into cephalic and 

 post-oral plates, the anterior being smaller and more sharply limited 

 than the other. In twelve hours more a second groove appears behind 

 the first, cutting off a narrow ridge, the first post-oral somite. At this 

 stage the embryo is readily comparable with Metschnikoff's pi. XVII 

 fig. 3, except in the following particulars: The two ends of the embryo 

 are more nearly equal, the single somite developed is much shorter 

 and the median groove is fainter and extends into both cephalic and 

 caudal plates. Successive somites are added by budding from the 

 caudal plate until the number six is reached. The embryo now closely 

 resembles Balfour's pi. XIX figs. 3 a and 3 è except that it covers far 

 less of the surface of the egg, the first somite is separate from the 



3 Quarterly Journ. Microsc. Soc. XXV. 1885. 

 * Quart. Journ. Microsc. Soc. XX. 1880. 



