560 WILLIAM PATTEN. 



his opinion, is a viscid fluid formed by the dissolution of the 

 yolk ; when it has covered the latter uniformly there appears 

 in it isolated white spots — the germ nuclei. These increase in 

 number, and finally cover the yolk with a continuous cell layer. 

 It is claimed here^ in accordance with the ideas prevalent at 

 that time, that the nuclei arise first, and that later a cell mem- 

 brane is formed around them. As soon as the blastoderm is 

 formed its cells are concentrated towards one side, thus leaving 

 the opposite side of the yolk covered by a thin layer, which 

 subsequently ruptures and allows the yolk to come in contact 

 with the vitelline membrane. The germinal band which 

 results from these changes occupies two thirds or three fourths 

 of the circumference of the egg. 



The second period, including the metamorphoses of the 

 germinal band and the development of the appendages, begins 

 with two important changes in the former. 1. Its separation 

 into two layers. 2. The formation of two germinal folds or 

 " Keimwulste " in the inner layer. The two layers are formed 

 by delamination in the head, and gradually extend to the 

 posterior end of the body. During the formation of these two 

 layers the germinal band not only becomes much thinner in 

 the mid-longitudinal line, but on the side towards the yolk a 

 deep furrow is formed by which the germinal band is partially 

 divided into two lateral ridges, the " Keimwiilste '^ thus at an 

 early period indicating the bilateral type and furnishing founda- 

 tion for the development of the segments and appendages. 

 The inner layer, after giving origin to the nervous system and 

 appendages, forms the muscles of the body walls, and hence is 

 called the " Muskelblatt.^-* The outer layer forms the external 

 covering of the sides and dorsum, the subsequently hardened 

 skin of the embryo, and hence is called the " Hautblatt.'^l 



The procephalic lobes are formed by the thickening of a pair 

 of lateral outgrowths from the " Hautblatt " in the head region. 

 After their formation and that of the oral invagination the 

 " Hautblatt " ruptures in the middle line throughout nearly the 



1 The "Hautblatt," without doubt, corresponds to the "amnion" and 

 serosa to^etlier. 



