404 OUTLINES OF CHORDATE DEVELOPMENT 



what shrunken and elongated, and the yolk stalk is clearly 

 distinguishable. Topographically the most anterior part of the 

 embryo is formed by the mid-brain, beneath which the fore- 

 brain is now folded back toward the heart. The heart is very 

 prominent and on the sides of the neck region three pairs of 

 gill clefts are indicated, decreasing in size posteriorly. Four 

 pairs of visceral arches (mandibular, hyoid, two branchial) are 

 thus marked out, and the most anterior (mandibular) already 



FIG. 164. Human embryo of 2.6 mm. From Minot (Laboratory Text-book 

 of Embryology), after His. The embryo is enclosed in the amnion and shows 

 the maxillary and mandibular processes, the rudiments of three gill clefts, and 

 the large heart. The large yolk-sac extends ventrally, while posterior to its origin 

 the root of the body stalk is shown turned dorsally. 



shows signs of its transverse division into upper and lower 

 portions, the maxillary and mandibular processes. Later a 

 fourth cleft and fifth arch are indicated. Of course in the 

 Mammal actual gill clefts are not present as perforations; the 

 so-called clefts are vestigial structures and, excepting the first, 

 merely form superficial grooves, opposite corresponding pockets 

 out of the pharyngeal cavity. An anterior depression, the 

 oral sinus (stomodaBum) between the mandibular arches, marks 

 the position of the future mouth, which is perforated very 

 shortly after this time. 



