1890.] Concrescence Theory of the Vertebrate Embryo. 509 
differently by some subsequent writers. The ectoderm, entoderm, 
and axial mesoderm Prof. His groups under the collective name 
of archiblast. This view 
of the double origin of 
the mesoderm, although 
it has been adopted in a re 
modified form by the p, fe 
brothers Hertwig, I am 
unable to accept. 
Concrescence în Mar- 
stpobranchs, Ganoids and 
Amphibians. — As not 
only the constitution of 
the ovum, but also its 
early development, is very 
similarinthe three classes Fic. s.—Ovum of axolotl; after Bellonci; lon- 
i gitudinal section to show the commencing formation 
named, we may consider of the primitive streak; 4/, blastopore ;- BZ, blasto- 
them collectively in the derm ; s.c., segmentation cavity. . oe : 
present connection. The condition of the ovum at the close of 
segmentation I have already described,’ p. 472 and figured, Fig. 
4,3. The ectental line is not sharply defined, nor does there 
. appear any groove around 
Pegs HdsOO AD» 
KLAR. the edge of the blastoderm 
a we as in meroblastic ova. The 
small-celled ectoderm 
spreads overthe yolk ; while 
it is doing this a small 
cavity appears at the hind 



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yk - edge of the blastoderm, 
eres 12) Y POS p, with a small opening to 
Eeeaong. ESS the exterior known as the 
Lo eiea blastopore, Fig. 5, d/.; the 
Frc. 6—Ovum of Petnmyzon in longitudinal Cavity runs forwards to- 
sectipes ater Balio. wards the segmentation 
cavity Fig. 5, sc; above and in front of the blastopore the 
cells have multiplied and accumulated to form the beginning 
3 AMERICAN NATURALIST, 1889. 
