WORK IN THE HATCHING-HOUSE. 51 



reared under cover, thus still further strengthening the reasons 

 in favour of grilles, and defeating the argument advanced against 

 them by foreigners, on the ground of the space they occupy. 



A few hours after the egg is laid down the germ rises. The 

 proportion of the germinal disc to the whole ovum is very small in 

 trout eggs, while in herring eggs the disc may form as much as 

 one-fifth of the whole. This will probably exercise a material bear- 

 ing on the attempts now being made to transport the ova of the 

 latter to New Zealand. The segmentation is meroblastic, and in 

 the ova of salmon and trout the first furrows only penetrate for a 

 certain depth through the germinal disc. " During the latter stages 

 of segmentation one end of the blastoderm becomes thickened, and 

 forms the embryotic swelling ; and a cavity appears between the 

 blastoderm and the yolk, which is eccentrically situated near the 

 non-embryotic part of the blastoderm. This cavity is the true 

 segmentation cavity. 



" With the appearance of the segmentation cavity the portion of 

 the blastoderm which forms the roof becomes thinned out, so that 

 the whole blastoderm consists of (1) a thickened edge, especially 

 prominent in one point where it forms the embryotic swelling ; and 

 (2) a thinner central portion. The changes which now take place 

 result in the differentiation of the embryonic layers, and in the 

 rapid extension of the blastoderm round the yolk, accompanied by 

 a diminution of its thickness. The first differentiation consists in a 

 single row of cells on the surface of the blastoderm becoming dis- 

 tinctly marked off as a special layer, which, however, does not 

 constitute the whole epiblast, but only a small part of it, which 

 will be spoken of as the epidermic layer. 



" The complete differentiation of the epiblast is effected by the 

 cells of the thickened edge of the blastoderm becoming divided 

 into two strata. The upper stratum constitutes the epiblast. It 

 is divided into two layers, viz., the external epidermic layer 

 already mentioned, and an internal layer, known as the nervous 

 layer, formed of several rows of vertically arranged cells. Accord- 

 ing to the unanimous testimony of investigators, the roof of the 

 segmentation cavity is formed of epiblastic cells only. The lower 



