

W. K. BROOKS ON THE GENUS SALPA. 265 



become irregularly pear-shaped, the broad end being, in nearly every 

 case, directed outwards, and the pointed end inwards. In most of the 

 elongated nuclei the chromatic substance is gathered at the inner end 

 in a mass which takes on a deep, diffused color from haematoxylin, while 

 the rest of the nucleus contains only a few scattered granules. Cell 

 multiplication, by direct division of the nucleus, now goes on so rapidly 

 in the visceral follicle cells that each section shows many nuclei in pro- 

 cess of division. 



The somatic layer soon becomes distended and separated from the 

 embryonic mass by a space which is shown, colored purple, at 15 in Plate 

 XI, Fig. 3, and also at 15 in cut A, page 29, which is a reconstruction 

 from a series of sections of Salpa hexagona at about the same stage as 

 Plate XI, Fig. 3, but at right angles to that figure. This space is the 

 body cavity of the salpa embryo, and it is marked 15 and colored purple 

 in the figures of the later stages. It is bounded on all sides at this stage 

 by follicle cells. 



The embryo now becomes complicated by several important changes 

 which will be described later on, although they must be briefly mentioned 

 here to make the later history of the follicle intelligible. The first of the 

 changes is the migration of the ectodermal blastomeres out of the follicle 

 into the position where they are shown at A" in cut B. They become 

 completely extra-follicular, and are covered only by the epithelial capsule 

 B'. They are shown in Salpa hexagona in Plate XI, Figs. 4, 5 and 7, 9', 

 and in Salpa pinnata, at 9' in Plate XII, Figs. 1 and 2 ; Plate XII, Figs. 

 2, 4, 5, a", and in other figures. In their migration to their final position 

 they pass through the part of the follicle where the two layers are con- 

 tinuous with each other; the part which is marked 10 in Plate X. 



The second complication is caused by the formation of the peri- 

 thoracic tubes, which arise as a pair of invaginations of the somatic 

 layer, shown at a very early stage in cut B at G". 



This cut was made from a series of actual sections, but they were 

 cut in such an oblique plane that it seemed better to combine them in a 

 diagram, than to draw a number of figures to illustrate a simple point. 

 No single section cut both of the tubes, but the relations were as shown 

 in the diagram. 



The perithoracic tubes push their way in across the body cavity into 

 the mass of visceral follicle cells, as is shown in cut B. This cut is not a 

 diagram, but a careful reconstruction in a vertical plane from the series 

 of horizontal sections of a young embryo of Salpa pinnata, shown in Plate 

 XVII, Fig. 5, and in the figures in Plate XII. 



