THE DEVELOPMENT OP THE MOLE. 107 



the sinus rhomboidalis. On either side of the embryo, just 

 behind the widely open anterior end of the medullary canal, a 

 ridge extends backwards and onwards over the blastodermic 

 vesicle ; these ridges are the first traces of the two tubes which 

 will eventually form the heart (compare fig. 5, ht.). 



Figs. 4 and 5 are two drawings of an embryo of about 

 the same age as that last described (Stage f). The length of 

 the latter is, however, greater than that of the former embryo, 

 being 2*12 mm., while the medullary groove is not so far ad- 

 vanced in development. My object in drawing fig. 4 is not 

 only to show these points but to represent the amnion, which 

 is as yet developed only at the hind end of the embryo^ 

 and has already grown nearly half way over the back of the 

 embryo. 



Fig. 5 is a transparent view of the same embryo, and indi- 

 cates the position of the first five protovertebrse, and of the 

 commencing tubes {ht.,ht.) which eventually will form the heart. 

 The blind lateral prolongations of the medullary groove at the 

 cephalic end are the optic grooves. In this figure also the 

 floor of the sinus rhomboidalis at its posterior end is seen to 

 contain a much thickened, forwardly projective knob, which, as 

 will be shown in sections, is the anterior end of the primitive 

 streak. The medullary folds may therefore be described as 

 extending posteriorly behind the front end of the primitive 

 streak. 



Stage G. — Stage g is represented by the embryo drawn in 

 fig. 6. The hinder portion of the medullary canal is much 

 the same as before; anteriorly, however, development has pro- 

 gressed, and the edges of the medullary folds have come 

 together and partially fused at the anterior end of the em- 

 bryo. At the extreme end, however, a pore is left, owing 

 to the more rapid growth of the sides than of the floor of the 

 canal as pointed out above. At this stage, therefore, the 

 neural canal is still open to the exterior, both anteriorly and 

 posteriorly. 



The optic grooves are now closed, and have given rise to the 



