REPRODUCTION. 67 
this region becoming more columnar. (histological differentia- 
tion). This depression (invagination) deepens until a cavity is 
Fie. 70.—Blastula and gestrula of amphioxus (Claus, after Hatschek). A, blastula with flat- 
tened lower pole of larger cells; B, commencing invagination; C, gastrulation completed!: 
the blastopore is still widely open, and one of the two hinder-pole mesoderm cells is seen 
at its ventral lip. The cilia of the epiblast cells are not represented. , 
formed (as when a hollow rubber ball is thrust in at one part 
till it meets the opposite wall), in consequence of which a two- 
layered embryo results, in which we recognize the primitive 
mouth (blastopore) and digestive cavity (archenteron), the 
outer layer (ectoderm) being usually separated from the inner 
(endoderm) by the almost obliterated segmentation cavity. 
Such a form may be provided with cilia, be very actively loco- 
motive, and bear, consequently, the greatest resemblance to the 
permanent forms of some aquatic animals. 
The changes by which the segmented odsperm becomes a 
gastrula are not always so direct and simple as in the above- 
described case, but the behavior of the cells of the blastosphere 
may be hampered by a burden of relatively foreign matter, in 
the form of food-yelk, in certain instances; so much so is this 
the case that distinct modes of gastrula formation may be rec- 
ognized as dependent on the quantity and arrangement of food- 
yelk. These we shall pass by as being somewhat too compli- 
cated for our purpose, and we return to the egg of the bird. 
' The Hen’s Egg.—By far the larger part of the hen’s egg is 
made up of yelk; but just beneath the vitelline membrane a 
small, circular, whitish body, about four millimetres in diame- 
ter, which always floats uppermost in every portion of the egg, 
may be seen. This disk (blastoderm, cicatricula) in the fertilized 
egg presents an outer white rim (area opaca), within which is 
a transparent zone (area pellucida), and most centrally a some- 
what elongated structure, which marks off the future being 
itself (embryo). All of these parts together constitute that por- 
tion (blastoderm) of the fowl’s egg which is alone directly con- 
cerned in reproduction, all the rest serving for nutrition and 
