142 EMBRYOLOGY. 



occurring elsewhere, in which organs that should be hollow are at 

 first developed as solid masses of cells. We shall hereafter cite as 

 such the solid fundament of the neural tube in Bony Fishes, many 

 sensory organs and the most of the glandular sacs, which latter 

 arise as solid buds of epithelial lamellae, and only later, when they 

 become functionally active, acquire a cavity by the separation of 

 their cells. 



SUMMARY. 

 A. The blastula. 



1. Out of the mass of cleavage-cells (morula) there is developed 

 in all Vertebrates a sac-like germ (blastula) with cleavage-cavity. 



2. There are four different kinds of blast ulae in Vertebrates, 

 according to the amount and distribution of yolk. 



(a) In Amphioxus the cleavage-cavity is very large, and its 



wall consists of a single layer of cylindrical cells of 

 nearly uniform size. 



(b) In Cyclostomes and Amphibia the cleavage-cavity is small : 



one half of the wall of the blastula is thin, and composed 

 of one or several layers of small cells ; the other half is 

 considerably thickened, and formed of large yolk-cells 

 arranged in many superposed layers. 



(c) In Fishes, Reptiles, and Birds (meroblastic eggs) the 



cleavage-cavity is small and fissure-like or wanting. 

 Only its roof or dorsal wall consists of cells (germ-disc) ; 

 its floor or ventral wall, on the contrary, consists of the 

 yolk-mass which has not been divided into cells, but 

 which contains yolk-nuclei in the vicinity of the margin 

 of the germ-disc. 



(d) In Mammals the cleavage-cavity is very spacious, and filled 



with an albuminous fluid ; its wall is composed of a single 

 layer of greatly flattened hexagonal cells, with the 

 exception of a small thickened place, where larger cells 

 in several superposed layers cause an elevation which 

 projects into the cavity. 



B. The cup-shaped larva or gastrula with two germ-layers, 



1. There ss, formed out of the blastula, by the invagination of 

 a portion of its surface, a two-layered form, the beaker-larva or 

 gastrula. 



2. The two layers of the double beaker are the outer and the 



