THE CLASSIFICATION OF THE EARLY STAGES. 115 



stage have been found and some of them accurately studied. They usually have 

 a remarkable bend in the back (Fig. 52), which imparts to the embryo a very 

 singular appearance. Nothing similar to this bend or dorsal flexure has been 

 observed in any other embryos. It has been held by His and others to be a 

 normal condition, and not the accidental result of a mechanical strain exerted 

 by the yolk-sac. If the condition is normal, it must exist for only a very brief 

 period, as it is not encountered in older or younger stages. We may suppose 

 if it is normal that the change from the concave to the convex position of the 

 embryo, as found in the next stage, is very abrupt. The head of the embryo 

 (Fig. 52) shows the characteristic head bend, and the tail end of the embryo is 

 also bent over ventralwards. The heart is large and very protuberant. It is 

 bent so that we can clearly distinguish the auricular, ventricular, and aortic 

 limbs- It shows distinctly its inner endothelial portion and outer mesoderm. 

 The yolk-sac extends from the heart backward to where the body of the embryo 

 turns to make the dorsal flexure. Between the heart and the head the oral 

 invagination has been formed, but is still separated by the oral plate from the 

 entodermic canal. Above the heart on either side is an open invagination of the 

 ectoderm, the anlage of the so-called otocyst, which in its turn is the anlage of 

 the epithelial labyrinth of the adult ear. In one embryo of this stage there were 

 shape found twenty-nine primitive segments. 



Ninth Stage. — Three Gill Clefts Showing Externally: This is, on the whole, 

 the best known of the early stages of human development. The embryos de- 

 scribed as belonging to it vary from 2.6 to 4.2 mm. in length. In one of them, in 

 which the embryo measured 3.2 mm., the chorionic vesicle measured 11 by 14 

 mm., and its supposed age was from twenty to twenty-one days. The general 

 of these embryos is indicated by figure 74. 



^ The head is bent down and the back is very convex. In figure 74 the 

 tail is rolled up and turned to the left. Usually, however, the tail turns to the 

 right and the head is twisted to the left, so that the long axis of the body 

 describes a large segment of a spiral revolution; the spiral form is marked in 

 embryos a little older. 



Tenth Stage. — Four Gill Clefts Showing Externally: There are no satisfactory 

 observations on this stage in man. 



Eleventh Stage. — Appearance of the Limb-buds: The embryo is much rolled 

 up, so that the head and tail overlap; four slight protuberances appear as the 

 beginnings of the limbs ; the cervical sinus is commencing by the invagination of 

 the posterior gill arches. 



