ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 661 



dentitional series. In the series to which the large multicuspidate 

 teeth of the adolescent animal belong the authors reckon five members. 

 They regard the epithelial nodules already described by Poulton and 

 observed by themselves as vestigial teeth, and see in them a whole series 

 of precursors of the molar teeth. These vestigial deciduous predecessors 

 constitute a series of simple tooth-rudiments, each on the whole corre- 

 sponding with one of the cusps of their multicuspidate molariform 

 successors. It is not necessary to suppose that any ontogenetic fusion 

 occurs. The mode of development of the successioual molars in the 

 younger stage is decisive against the occurrence of any fusion process. 

 But the relation of the two series is suggestive of some sort of phylo- 

 genetic substitution of a small number of compound teeth for a large 

 number of simple teeth. Further, no support is found for any theory 

 which would seek to establish a serial distinction between the true molar 

 teeth and those which appear, prima facie, to belong to the same series 

 in front of them. 



b. Histology. 



Structure of Smooth Muscle.*— C. McGill has studied the structure 

 of the smooth muscle of the intestine in the contracted state, utilising 

 various mammalian and amphibian types. In all the forms studied it 

 is a syncytial structure. In an area of contraction one or more thickened 

 apparently homogeneous nodes appear, the internodal segments (which 

 may be partially contracted) and the entirely relaxed muscle stain more 

 lightly and are distinctly fibrillated. Even in the deeply staining con- 

 traction nodes it is possible to demonstrate the myofibrillae, which take 

 an active part in the contraction process. During contraction the 

 smooth muscle nuclei shorten and thicken by an active process. The 

 chromatin collects chiefly at the two ends of the nucleus, leaving a 

 relatively clear area in the centre. 



Organogenesis of the CEsophagus.f— J. M. Flint gives an account 

 of the organogenesis of the oesophagus in the lower segment about the 

 level of the forking of the trachea. The mucosa is derived from the 

 simple epithelium of the head gut, which, in the early embryos, consists 

 of a single or double layer of cubical or columnar epithelium. The 

 trachea and oesophagus are practically separate from the head gut in 

 pig embryos of 6 mm. long. The submucosa is developed primarily 

 from the mesoderm about the head gut. In the early stages this con- 

 sists of a syncytium of anastomosing cells which differentiate connective- 

 tissue fibrils. These fibrils give rise to the trabeculse and form the 

 framework of the oesophagus, of which the submucosa is chiefly formed. 

 The muscularis mucosfe in embryos of 7 ■ 5 cm. consists of loosely packed 

 cells lying well outside the basement membrane. Subsequently they 

 form an indefinite layer composed of smooth muscle fibres lying between 

 the mucosa and the tunica muscularis. The tunica muscularis appears 

 in embryos of 13 mm. as a densely packed group of cells, well outside 

 the mucosa. In embryos of 7 • 5 cm. they form two more or less well 



* Anat. Anzeig., xxx. (1907) pp. 426-33. f Tom. cit., pp. 442-51. 



2x2 



