112 ^I. Ussow's Zoologico-Embryological Investigations. 



also the first <:;onii-lamclla is thickened (especially at the sides 

 and on the dorsal surface) by its cells dividing rapitlly in the 

 longitudinal direction, by which means they become higher 

 and cylindrical. 



This ])art is followed immediately by the rather narrow 

 girdle-like i)art, originating from the segmentation-sj)heres 

 unifonnly separated by constriction from all the tliirty-two 

 segments, which occupies the equator of the vitellus, and is 

 bounded above by the middle portion (the future truidc of the 

 animal), and below by the blastoderm (rudiment of the yelk- 

 sac), which is everywhere uniformly thin, two-layered, and 

 closed at the inferior pole. In the above-mentioned peripheral 

 girdle-like portion of the spherical germ the cells are broad, 

 but at the same time flat ; so that this part is as tliin as the 

 rudiment of the yelk-sac. At the end of the iirst period all 

 the cells of the upper germ-lamella are distinguished only by 

 their height and breadth ; as regards their form there is nothing 

 peculiar to certain parts of this germ-lamella. There are cells 

 with three or four angles, and with them others with five, six, 

 seven, or even eight angles. 



2. The second or middle germ-lamella, which attains its 

 gi'eatest thickness at the oval ibid, and splits into two layers, 

 the dermo-museular layer and the intestino-fibrous layer. 

 With the development of the germ this cleavage of the 

 middle germ-lamella increases both by the transverse division 

 of its cells and also by the spreading of the two layers, which 

 takes place in the direction from the rhomboidal centre towards 

 the yelk-sac. 



The two layers of the second germ-lamella show the follow- 

 ing characters : — a. The dermo-museular layer {Hmitmuskel- 

 schicht) thickens somewhat in the central part of the blasto- 

 derm and in the girdle-like ring situated on the equator of 

 the vitellus ; by the continued gradual division of the cells of 

 the blastoderm (sec the commencement of the formation of the 

 second germ-lamella), and by the indej)endent longitudinal 

 division of its cells, this layer grows pretty ra})idly beneath the 

 upper germ-lamella and becomes closed at the inferior pole of 

 the nutritive vitellus. h. The intestino-fibrous layer {Darvi- 

 fasersch'cJit), as the development of the germ goes on, occurs 

 not only on the ventral surface (below the oval annular fold 

 of the rhomboidal centre), but its rather loose cell-series, lying 

 immediately upon the nutritive vitellus, also increase towards 

 the dorsal surface in the middle part of tlie germ. Various sec- 

 tions from earlier stages {e. g. of the tenth day) show that 

 the cells of the intestino-fibrous layer accumulate most on the 

 sides of the longitudinal axis of the germ, namely where the 



