80 J. BEARD, 



The embryo was cut up into transverse sections, the head being 

 cut separately from the rest. 



In section. There is a well-marked dental ridge in both upper 

 and lower jaw , and this has grown some distance inwards , but no 

 tooth-germs are as yet diiferentiated. The degree of tooth develop- 

 ment is nothing like so advanced as in the older embryo (B). 



The tongue is rather rounded in section, in which respect it differs 

 from that of the second specimen. 



Jacobson's organ is formed and the choana is open. The lach- 

 rymal duct is a solid rod, which does not open into the nasal chamber. 



Hair-folUcles are present under the thick outer layer of the epi- 

 dermis (epitrichium). 



The eye. The lens has a thick anterior wall, and the cells of 

 its posterior wall are elongated, but no formation of lens-fibres has 

 yet taken place. The retina is about half pigmented. 



The external ear is formed, but is entirely covered over by the 

 thick epitrichium. The external auditory meatus is filled with epi- 

 dermal cells. Sacculus and utriculus are present, and the semicircular 

 canals are formed. 



The nasal cartilages are differentiated, and there is a cartilaginous 

 lower jaw. 



There is as yet no budding in the hypophysis. The epiglottis is 

 well-developed. 



The thymus is developed. The clavicles are well-ossified at their 

 outer ends and, partly, in the middle. There is no ossification at the 

 inner ends, which are represented by a dense cell-mass. There is a 

 cartilaginous sternum, and cartilaginous ribs are laid down. 



The skeleton of the anterior limb is cartilaginous, and there are 

 well-marked claws on the digits of the hand. 



The red blood-corpuscles are, of course, nucleated. In the heart 

 the foramen ovale is an open network. The lungs are well-ad- 

 vanced, and there are alveoli, but they naturally occupy far less space 

 than those of the newly-born foetus. 



The notochord is very small in section. There are well-formed 

 cartilaginous vertebral centra and cartilaginous neural arches ^). 



The diaphragm is still somewhat incomplete posteriorly. The 

 oesophagus has a complete lumen, stomach and pancreas are formed. 



1) Posteriorly, i. e. behind the thoracic region, neural arches are 

 not so well- formed. 



