Miscellaneous. 129 



with the unsegmented central yolk-mass, but in part separates 

 entirely therefrom. With further multiplication of the blastoderm 

 cells the division in the yolk diminishes and soon completely disap- 

 pears. Finally we get a blastula stage, consisting of a superficially 

 situated uniform layer of cells and an internal yolk-mass. 



I must here remark that at this time some of the cells mi- 

 grate into the yolk and give rise to a small number of yolk-cells 

 (vitellophaga). These latter undergo a regressive metamorphosis 

 and soon disappear almost entirely. 



The rudiment of the embryo subsequently appears on the ventral 

 side of the ovum in the shape of a thickening of the blastoderm. 

 In the region of this rudiment there may be observed three separate 

 swellings or thickenings — a posterior thickening in the shape of 

 small groups of cells (caudal section), and, a little later, the ante- 

 rior paired rudiment of the optic lobes. 



The formation of the germinal layers commences very early from 

 the cells, which, in consequence of the speedy multiplication of the 

 blastoderm cells, appear in the region of the posterior thickening. 

 It must be remarked that the entrance of the cells into the yolk 

 takes place either in the shape of compact masses or of a sac-shaped 

 depression. 



On further multiplication the cells of the posterior thickening 

 become dispersed in the shape of a fan, for many of them pene- 

 trate deep into the yolk, while others, fewer in number, spread 

 out ou the ventral surface. The former, on all sides separating 

 equally one from another, commence to permeate the yolk. Sub- 

 sequently, on the appearance of the thoracic appendages, all these 

 cells pass through the yolk and collect their scattered units on the 

 surface of the yolk-mass. They form the commencement of the 

 endoderm layer. 



As regards the middle layer, this is also formed from the above- 

 mentioned cells of the posterior thickening, which spread out upon 

 the ventral surface. To these must further be added those cells 

 which arise, owing to the multiplication of the ectoderm cells, upon 

 the lateral thickenings of the embryo. The mesoderm cells at first 

 lie upon the ventral surface, and collect for the most part in the 

 protuberances, which subsequently give rise to the limbs. On its 

 first appearance the mesoderm layer has a paired structure, and 

 consists of cells arranged in two longitudinal rows. 



The nervous system already commences in the Nauplius-9,tQ.ge to 

 develop from in front backwards in the shape of paired thickenings 

 of the ectoderm. In the stage which is just commencing to lead an 

 independent existence we may count eighteen ganglia, of which the 

 last is double. 



The eyes are developed from the anterior thickenings of the ecto- 

 derm, which for a very long time present the appearance of an 

 undifferentiated aggregation of cells ; and it is only at the period of 



Ann. (& Mag. N. Hist. Ser. 6. Vol, xv. 9 



