A TAXONOMIC STUDY ON PYEOSOMA METCALF AND HOPKINS. 209 



portion of this disk (fig. 6). This definitive blastoderm gives rise 

 to two thin layers, one of ectoderm cells, and another of mes-endo- 

 dermal nature beneath it, both lying above the yolk. A cavity then 

 forms in the mes-endodermal mass of cells, which corresponds prob- 

 bly to the archenteron. Certain cavities appearing in the axial 

 mesoderm have been referred to as representing the notochord. 



The organs of the embryo form very much as they do in other 

 Tunicates (figs. 6 and 7). A neural rudiment (n. r.) is defined by the 

 infolding of an ectodermal thickening. There is formed also a ciliated 

 funnel (c.f.) communicating with the neural rudiment by a short duct. 

 Two invaginations, one on each side of the neural rudiment, push 

 forward for a considerable distance as peribranchial pouches (pbr.). 

 Their apertures grow together later behind the ganglion, forming a 



^'--\ 



Fig. 6.— An embryo of Pyrosoma giganteum (?). After 

 kowalevsky (1875). 



common cloacal aperture (cl.a.) . The mesoderm gives rise anteriorly 

 to a layer of cells inclosing a paired coelomic space, and this to a pair 

 of lateral outgrowths. The right one of these, only, develops a lumen, 

 the pericardial canal, which swells into a pouch distally. This swollen 

 portion becomes separated off as the future pericardium, and the 

 original canal degenerates. 



The embryo, at this stage of its development, contains an archen- 

 teron, undifferentiated mesoblast cells, a pericardium, a nerve gang- 

 lion and ciliated funnel, and peribranchial and cloacal chambers. 

 The energy is now thrown into the formation of a proliferating stolon 

 and the subsequent production of buds. The embryo, from this 

 time on, ceases to become more differentiated, except in a few particu- 

 lars. A functional circulatory system is evolved, by means of which 

 the yolk nutriment of the embryo is transferred to the growing 

 buds. This first individual, then, which developes from the egg, 

 7911—19 2 



