ON THE ORIGIN OP VERTEBRATES FROM AKICHNIDS. 373 



derm, is split off from the blastodisc before the germ-baud is 

 formed. 



In surface views of the germ-band of Scorpio, we see in 

 the median line at the posterior end of the body a great 

 thickening, from which cells grow forward and laterally. From 

 the median portions of the band of tissue thus formed arise 

 the sexual organs and the botryoidal cord, and from the more 

 lateral portion the mesoblast and endoderm. 



In Limulus there is, at the posterior end of the embryo, a 

 short, but distinct slit-like invagination, which, in surface views 

 and in sections, is exactly like the primitive streak of many 

 birds and reptiles (Fig. 18, b and d). 



The lateral edges of the wedge-shaped mass of cells pro- 

 duced by this invagination spread out on either side between 

 the yolk and the ectoderm to form poorly defined mesodermic 

 bands. At irregular intervals, but principally near the primitive 

 streak, great masses of cells wander inwards from the inner 

 layer cells, and are scattered throughout the yolk. 



Moreover, in a number of different places great masses of 

 mesoderm and endoderm cells may be proliferated inwards 

 from the ectoderm of the ventral plate, in a manner very 

 similar to what occurs in the primitive streak ; the proliferating 

 points differ from the primitive streak in being nearly trans- 

 verse instead of longitudinal, and in not being accompanied 

 by any overlying depression of the ectoderm^ (J^ig. 18, 

 A x> x) • These facts prove conclusively, I think, that the 

 formation of mesoderm and endoderm in the ventral plate of 

 Limulus does not follow any method which can possibly be 

 regarded, in the concrescence theory or any other, as a modifi- 

 cation of a gastrula. I have examined a great many embryos, 

 and have never seen any traces of a median furrow connecting 

 the mouth and anus such as is described by Kings ley, and 



1 It is possible tliat in some places at least these proliferating points may 

 be cross sections of the somites, which are much curved at the posterior 

 end of the body. I have seen indications in young scorpion embryos that 

 some somites are formed by a distinct transverse proliferation 

 of the ectoderm. 



