438 GASTRULATION 



appears at the caudal edge of the embryonic shield; this opening forms the 

 dorsal lip of the blastopore (figs. 210A; 211 A, B). 



In teleost fishes with a primary hypoblast arranged as in Fundidus (fig. 

 180A, B), as the entodermal cells of the hypoblast move anteriad from the 

 deeper portions of the blastodisc, the entodermal cells exposed at the caudal 

 edge of the epiblast move over the blastoporal lip (i.e., involute) and migrate 

 forward as a part of the entoderm already present in the deeper layer. (See 

 arrows, fig. 180B.) The primary hypoblast thus becomes converted into the 

 secondary hypoblast. In teleosts with a primary hypoblast or entodermal ar- 

 rangement similar to Salmo (fig. 181 A), the secondary hypoblast is formed 

 by the forward migration and expansion of the entodermal mass located in 

 the caudal area of the embryonic shield. In both Fundulus and Salmo follow- 

 ing the initial forward movement of the entodermal cells, the pre-chordal plate 

 cells together with the notochordal cells move caudally and involute over the 

 dorsal blastoporal lip, passing to the inside. (See arrows, figs. 180B; 18 IB.) 

 The pre-chordal plate and notochordal cells migrate forward along the midline 

 of the forming embryonic axis. The pre-chordal plate cells lie foremost, while 

 the notochordal cells are extended and distributed more posteriorly. The 

 presumptive mesoderm in the meantime converges toward the dorso-lateral 

 lips of the blastopore (figs. 180B; 18 IB, see arrows), where it involutes, 

 passing to the inside between the entoderm or secondary hypoblast and epiblast. 

 Within the forming gastrula, the mesoderm becomes arranged along the upper 

 aspect of the entoderm and on either side of the median, notochordal material 

 (fig. 210E, F). The mesoderm in this way becomes inserted between the 

 flattened entoderm (secondary hypoblast) and the outside ectodermal layer 

 (Oppenheimer, '36; Pasteels, 36; Sumner, '03; Wilson, 1889). 



During the early phases of gastrulation, the involuted entodermal, noto- 

 chordal, and mesodermal tissues may superficially appear as a single, thick- 

 ened, cellular layer. As gastrulation progresses, however, these three cellular 

 areas separate or delaminate from each other. When this separation occurs, 

 the notochordal cells make their appearance as a distinct median rod of cells, 

 while the mesoderm is present as a sheet of tissue on either side of the noto- 

 chord. The entoderm may form two sheets or lamellae, one on either side 

 of the notochord and below the mesodermal cellular areas (fig. 21 OF) or it 

 may be present as a continuous sheet below the notochord and mesoderm 

 (fig. 210E, H). The entodermal lamellae, when present, soon grow mediad 

 below the notochord and fuse to form one complete entodermal layer (Wilson, 

 1889). 



2) Epiboly. Emboly involves for the most part the movements of cells in 

 the caudal and caudo-lateral areas of the blastoderm, i.e., the embryonic 

 portion of the germ ring. However, while the involution of cells concerned 

 with the development of the dorsal, axial region of the embryo occurs, the mar- 

 gins of the blastodisc beyond the dorsal-lip area, that is, the extra-embryonic, 



