60 EDWARD MCCRADY, JR. 



was very badly handled before sectioning*. During an in- 

 cautiously rapid dehydration the vesicle collapsed so that the 

 sections are wrinkled and in many cases broken. In all of 

 these repects the material is unsatisfactory, but as no speci- 

 men of this stage has been described before, and as no other 

 material is available at present, it seems worthwhile to in- 

 clude it tentatively until better material can be obtained. 



Fortunately the drawing of the external view was made 

 before the vesicle collapsed. This is shown as stage 23 in 

 the second normal stage plate. It can be seen that there are 

 four well-formed somites. They lie beneath the medullary 

 plate which is now easily recognizable even in a whole mount. 

 Lateral to the medullary plate the parietal mesoderm forms a 

 dark shadow. Anteriorly (i.e., in the region cranial to the 

 somites) the parietal mesoderm has split to form the two 

 coelomic cavities, which at this time do not extend beyond the 

 pericardial region. There are no heart tubes recognizable in 

 this specimen though the splanchnopleure is very decidedly 

 thickened throughout the pericardial region, and must have 

 been prepared to fold into myocardial tubes very soon. 



No sensory anlagen are recognizable. The ectoderm is be- 

 ginning to fold under the anterior edge of the medullary 

 plate (the subcephalic fold), but the mesoderm is still con- 

 tinuous across the midline in front of the embryo so that there 

 is no pro-amnion. 



In the posterior fifth of the medullary plate the primitive 

 groove is well marked. Immediately caudal to it and outside 

 the medullary plate a light mark represents the spot where 

 the ectoderm folds in slightly to form the anal pit or proc- 

 todeum. 



IX. THE NINTH DAY. Stages 24 to 26 



The first third of the ninth day 



Stage 24. The heart tubes and the first blood vessels. The 

 first sensory anlagen. The nephrogenic ridge. The first 

 branchial pouches. The hypophyseal plate and the pharyn- 

 geal membrane. The proamnion. Differentiation in the 



