Nomenclature of Chicken Embryos. 25 



two olfactory pits are widely open, and a groove from each 

 leads down to the mouth. The fronto-nasal process is not 

 yet well marked. 



Stage Q. — The gill-clefts are disappearing. The mid 

 brain forms the most anterior and prominent part of the 

 head. The limbs form prominent blunt processes, of which 

 the anterior are the longer. There is no appearance of 

 digits. The heart is enclosed within the body wall. The 

 eye is deeply pigmented, and the choroid fissure prominent. 

 The allantois stretches round to the dorsal surface. The 

 body is curved, so that the posterior limbs almost touch the 

 head in the region of the cerebral hemispheres. The fronto- 

 nasal process is broad, and the olfactory pits widely open. 



Stage B. — The gill-clefts have disappeared. The cerebral 

 hemispheres have grown, and the mid brain is less 

 prominent than in the previous stage. The eye is promi- 

 nent, and the choroid fissure is not very distinct. The 

 fronto-nasal process begins to be prominent when seen from 

 the side. The anterior limb is curved, so that the ])roximal 

 and distal halves form a sharp angle with each other, the 

 elbow being thus indicated ; the extremity of each limb is 

 flattened out, but there is no clear indication of digits. The 

 tail is clearly marked. The olfactory pits are slit-like, and 

 the mouth much smaller than in the preceding stage. 



Stage S. — The head has increased in size very markedly, 

 relatively, to the rest of the body. The eye is very large, 

 and the nictitating membrane appears as a fold. The fronto- 

 nasal processes begins to be drawn out into a beak-like 

 structure, and has almost, if not quite, united on either side 

 with the maxillary process, leaving only the tvt^o external 

 nares leading into the olfactory pits. The hind limb shows 

 a well-marked bend, and is longer than the anterior. The 

 anterior limb shows two blunt processes at its extremity, a 

 larger and a smaller, and the posterior limb shows traces of 

 four digits. 



Stage T. — The rudiments of feathers appear m the form 

 of a series uf rows of knob-like processes on the head, 

 back, ventral surface, tail, and the proximal part of the 

 limbs. The beak is well marked, and has a small whitish 

 knob on its upper surface. The anterior limb shows very 

 clearl}^ at its extremity, a smaller and a larger process, the 

 latter being divided into two digits. Four digits are very 

 distinct in the posterior limb. 



