ONTOGENETIC RELATION OF ALLIED FOEMS. 



67 



between systematically (and hence also phylogenetically) 

 allied forms to Man, and if, with reference to this law, we 

 rapidly run through the earliest human conditions, the first 

 striking thing noticeable in the early history of the germ 

 is the morphological identity of the egg-cells of Man and 

 of other Mammals (Fig. 1). All the properties that cha- 

 racterize the mammalian egg, are also observable in the 

 human egg; especially that characteristic structure of its 

 coating (the zona pellucida) which clearly distinguishes 

 the mammalian egg from that of all other animals. When 



Fig. 121. — Lyre-shaped germ-shield of a dog. 

 '' Double shield " of Remak, " embryonic rudiment ' 

 of other authors.) The dorsal furrow is visible in the 

 centi'e ; on either side are the medullary swellings. 



the human embryo is fourteen days old, 

 it, like all other mammalian embryos, is 

 in the form of ' an entirely simple, lyre- 

 shaped germ-shield. Along the middle 

 line of the dorsal side of this, there ap- 

 pears the rectilineal, groove-shaped medul- 

 lary furrow, bordered by the two parallel 

 dorsal, or medullary swellings. The ventral side is attached 

 to the wall of the globular intestinal germ- vesicle. In this 

 stage the human embryo is one line, or two millimetres 

 in length. It is not distinguishable from that of other 

 Mammals, e.g., of the Dog (Fig. 121).io2 



A week later, or at the end of the twenty-first day, the 

 human embryo has already attained twice this length : it is 

 now two lines or about five miUimetres in length, and already 

 shows, when seen from the side, the characteristic curvature 

 of the back, the swelling of the head end, the earliest rudi- 



