GASTRULA THEORY 



47 



The question here arises, What is the fate of the primitive streak ? Is it converted into the 

 embryonal axis as it increases in length, or is the growth of the embryo effected by a continuous 

 proliferation from a growth-centre at the head end of the streak ? Experimental data seem to 

 show that the portion of the blastoderm in front of the streak gives rise to the extreme head end 

 of the embryo, that the anterior end of the streak forms the rest of the head, while- the remainder 

 is converted into the trunk, the growth-centre being placed near its posterior end, and forming 

 ultimately the knob of undifferentiated blastema which gives rise to the tail. 



FIG. 70. LONGITUDINAL SECTION THROUGH THE MIDDLE LINE OF PART OF AN EMBRYONIC AREA IMOLEI 



IX WHICH THE PRIMITIVE STREAK HAS BEGUN TO FORM. (Heape.) 



The blastoderm is perforated in front of the (short) primitive streak (? blastopore, blp) ; a few 

 mesoderm-cells are seen anterior to the perforation ; ec, ectoderm ; en, entoderm ; p.s., primitive 

 streak. 



Starting from these premises, and accepting the assumption that the streak represents 

 the nrastrula-mouth drawn out, the axial increase from before backwards has been interpreted 

 as signifying the fusion of the lips of the blastopore postulated by the concrescence theories of 

 His. Minot, and Oscar Hertwig. According to this conception, the bilateral symmetry of the 

 vertebrate has been brought about by the elongation of the radial gastrula into a cylinder, and 

 the fusion along the dorsal aspect of the lips of the gastrula-mouth. The fusion takes place from 

 before backwards, and is manifested by the apparent backward growth of the dorsal lip of the 

 blastopore, as the embryonal axis is laid down in front of it. This process results in the closing-in 



cct 



FIG. 71. SECTIONS ACROSS AN AMPHIOXUS EMBRYO. (Hatschek.) 



.f/., neural groove ; n.c., neural canal ; ch, rudiment of notochord ; mes. sow., mesodermic somite. In 

 I. its cavity is in free communication with the alimentary cavity ; ect, ectoderm ; ent, entoderm ; 

 at, alimentary cavity. In III. the cavity of the somite has extended on either side of the alimentary 

 canal and forms a ccelom, or body-cavity (cte). 



of the archenteron, the roof of which forms the dorsal aspect or axis of the embryo, with the 

 notochord and segmented mesoderm. 



A considerable tide of opinion has in recent years set in in favour of a somewhat modified 

 conception of gastrulation. Keibel and Hubrecht ' in 1888 independently worked out the con - 

 ception of gastrulation in two phases; Hubrecht in 1902 named these two stages kephalogenesis 

 and notogenesis. The primary gastrula, formed in all Craniota by delamination, has a radial 

 symmetry and forms the fore-part of the head. The second stage of ontogeny, embracing the 



See Hubrecht and Keibel, Quart. Jour. Micro. Sc. xlix. 1905. 



