THE HUMAN EMBEYO AT DIFFERENT PERIODS. 



75 



the length of the embryo is 1'9 ram., the neural folds are well developed, the neural 

 groove is still completely open, and six pairs of mesodermal somites are visible (Fig. 97). 



In the next few days the length increases to 2 '5 mm., the neural groove closes except 

 in the cranial and caudal regions, the number of mesodermal somites is increased to four- 

 teen pairs, and the cranial region begins to bend ventrally as the cervical flexure forms 

 (Fig. 98). 



By the end of the first month the greatest length of the embryo is about 2'6 mm., 

 the head is bent at right angles to the body, the Wolffian ridges have appeared along 

 the ventral margins of the mesodermal somites and indications of the limb rudiments 



FIG. 95. FRASSI'S ZYGOTE. Estimated to be 18-19 

 days old (Bryce). The embryonic area is 1*17 

 mm. long and '6 mm. broad. Copied from Nor- 

 maltafeln, Keibel and Elze, representing a recon- 

 struction. The chorion is not shown. The upper 

 part of the amnion is cut away, and the dorsal 

 aspect of the embryonic area is seen from above. 

 In the centre of the area is the neurenteric 

 canal and caudal (inferior in the Fig. ) to it is the 

 primitive groove. Cephalwards of the neurenteric 

 canal is the neural groove, in the middle of the 

 neural plate. At the lower (caudal) end of the 

 Fig. is seen a section of the body stalk containing 

 the allantoic diverticulum, and the nodulated 

 area seen at the upper and right lateral part of 

 the Fig. is a portion of the yolk-sac. 



FIG. 96. SPEE'S ZYGOTE. (From Keibel and Elze's 

 Normaltafeln.} Length of embryonic area 1'54 mm. 

 Estimated age 19-20 days (Bryce). At the lower 

 end of the Fig. (caudal end of the embryo) is seen 

 a portion of the chorion attached to the embryo by 

 the body stalk. A portion of the amnion is still 

 attached to the margin of the embryonic area, and 

 the dorsal surface of the embryonic area is exposed. 

 In the median plane of the area is the neural groove, 

 and at the caudal end of the groove is the neur- 

 enteric canal. The caudal part of the area is bent 

 ventrally, and upon it is the remains of the primitive 

 groove. The yolk-sac is seen at the upper and 

 right part of the Fig. 



are present. The rudiments of the otic vesicles have appeared as slight depressions in 

 the region of the hind-brain. The anterior and posterior neuropores are still open 

 (Fig. 99). 



In the latter part of the fourth or the beginning of the fifth week the embryo attains 

 a length of about 5 mm., when measured from the vertex of the head to the base of the 

 tail, the mesodermal somites increase to thirty-five ; the rudiments of the fore- and hind- 

 limbs become quite distinct; the otic vesicles sink into the interior of the head but 

 remain connected with the surface by the recessus labyrinthi, the tail becomes a very 

 definite appendage, and the bulgings caused by the otic vesicles are quite obvious on 

 the surface of the head. The cervical flexure remains acute, and the head bends at right 

 angles upon itself in the region of the mid-brain, forming the cephalic flexure, with the 

 result that the frontal extremity of the head is turned caudally (Fig. 100). 



By the end of the fifth week the length of the embryo has increased to 1 1 mm. (OR) l 



1 CR indicates the crown-rump or crown-breech measurement which corresponds with the sitting 

 height (Mall). 



