THE CLASSIFICATION OF THE EARLY STAGES. 121 



Seventh Stage. One Gill-cleft Showing Externally: Not known by observation. 



Eighth Stage. Two Gill-clefts Showing Externally: Several embryos in this stage 

 have been found and some of them accurately studied. They usually have a re- 

 markable bend in the back (Fig. 67), 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. 67) shows the character- 

 istic head-bend, and the tail end of the embryo is also bent over ventralward. 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 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 described 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 n by 14 mm., and 

 its supposed age was from twenty to twenty-one days. The general shape of these 

 embryos is indicated -by figure 89. 



The head is bent down and the back is very convex. In figure 89 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: The internal gill-pouches 

 reach the ectoderm, and for each there arises a corresponding external depression 

 that of the fourth arch is often indistinct; hence this stage is more easily recog- 

 nized by the beginning of the limb-buds. A good embryo near the end of this 

 stage has been carefully studied by Broman. 



Eleventh Stage. Open Cervical Sinus: The cervical sinus is formed by the invagi- 

 nation of the ectodermal area of the fourth and fifth and later also of the third 

 gill-arches. The deep depression thus formed lasts for some time, but closes over 

 ultimately (embryos of 10 mm.). The eleventh stage comprises a relatively long 

 period. 



