MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 79 



GASTRULATION AND FURTHER CHANGES IX THE FORM OF THE LARVA. 



In the blastula, which has just begun to invaginate, the invagination is eccentric, thus giv- 

 ing the firal indication of the bilateral symmetry of the larva. This is further emphasized in 

 the young gastrula of Phoronis architecta by a thickening of the ectoderm cells, which becomes 

 the ganglion of the Actinotrocha. The cells composing this thickening of the ectoderm are 

 at the apical pole in the blastula and they bear long cilia, but as gastrulation takes place and 

 as the embryo elongates the thickening comes to occupy a position nearer the anterior end of 

 the larva (figs. 15, L5a, 155). The published accounts seem to indicate that the ganglion 

 makes its appearance much later in most specie- than it does in Phoronis architecta, although 

 Koule (20) figures the "plaque cephalique" at a rather early stage in Phoronis sahatieri. The 

 changes which take place in the shape of the blastopore are much like those described by 

 other investigators. At the beginning of gastrulation the blastopore is wide open and circular in 

 outline. The lateral lips of the blastopore then gradually draw together in the posterior region, 

 inclosing that part of the wall of the archenteron between them (fig. 20c), which becomes a 

 solid mass of cells continuous with the cells of the fused lips of the blastopore (figs. 18<2, 18c). 

 The cells of the solid mass are of the same character as those of the wall of the archenteron 

 where the blastopore is open. They are quite granular, except at the periphery, where they 

 project into the cavity of the blastoccele, and their nuclei are quite indistinct. Both of these 

 facts are characteristic of the cells, making up the archenteric wall (fig. 18c). 



As a result of the closing up of the blastopore posteriorly, the blastopore becomes oval in 

 shape, and an indication of a ventral furrow, first observed by Caldwell and called a 'primitive 

 groove. - * appears. In l'}i<>r<>n'ix architecta this groove is only to be seen in one or two sections 

 back of the blastopore, after which the ventral surface is convex (tigs. 18<2-20c). A "primitive 

 streak," as described by Caldwell (3a) could not be made out. The gastrula, which is at first 

 circular in horizontal section, becomes slightly elongated when the blastopore takes on aivoval 

 shape. 



Gradually the blastopore lips close up more anteriorly until the blastopore becomes circular 

 in outline but much smaller than it was originally. At the same time the anterior end of the 

 larva begins to bend in a ventral direction and the archenteron becomes elongated posteriorly 

 (fig. 20). 



Xow the larva increase- slightly in length (tig. 21), the blastopore assumes the form of a 

 transverse slit, the anterior end bends farther ventrally, and the posterior end of the enteron 

 becomes applied to the ectoderm at the posterior end of the larva (tig. -11). 



Our observations on Phoronis architecta agree with the description of Masterman (Hi). Ikeda 

 (9), and Longchamps ( 12) in regard to the closure of the lips of the blastopore and the resulting 

 change in the shape of the latter, but in Phoronis <ir<-]iit,ct<i the definitive blastopore does not 

 seem to be pushed farther anteriorly by the special activity in the posterior region of the blast- 

 opore, as Ikeda has found to be the case for Phoronis igimai. The definitive blastopore seems 

 to be represented by the anterior part of the wide circular blastopore of the young gastrula. Its 

 change in position with reference to the anterior end is due to an elongation of the posterior 

 portion of the embryo and the ventral flexure of its anterior end. 



Our studies on the development of Phoronis architecta lead us to agree with Ikeda and 

 Longchamps as to the fate of the cell- in the posterior part of the blastopore and as to the ecto- 

 dermal origin of the " posterior pit." The cells of the posterior part of the blastopore become 

 invaginated by the closure of the blastopore lips and form part of the ventral wall of the enteron. 

 while the '" posterior pit." which appears in Phoronis architecta shortly before the definitive 

 blastopore is formed, is of ectodermal origin and has no apparent relation to the ventral groove. 

 A- Ikeda (9) has stated, tin' pit is the beginning of the nephridium of the Actinotrocha. In a 

 recent paper Masterman ( L6a) says that he has found the " posterior diverticulum" both in larva' 

 of /'. buskii and /'. hippocrepia and that he considers them to be the anlagen of the nephridia. 



