438 NUMMARY OF CURRENT RESEARCHES RELATING TO 



tion, the egg will be compressed in the middle, and will tend more 

 or less to the form of a cylinder with spherical ends. The eggs of 

 the grebe and cormorant (or crocodile) may be supposed to receive their 

 shape in such circumstances. 



When the egg is subject to the peristaltic contraction of the ovi- 

 duct during its formation, then from the nature and direction of 

 motion of the peristaltic wave the pressure will be greatest somewhere 

 behind the middle of the egg ; in other words, the tube is converted 

 for the time being into a more conical form, and the simple result follows 

 that the anterior end of the egg becomes the broader and the posterior 

 the narrower. 



In an egg, consisting of an extensible membrane filled with an 

 incompressible fluid and under external pressure, the equation of the 



envelope is p„ + T (— +—\ = P, where p n is the normal component of 



external pressure at a point where r and r 1 are the radii of curvature, 

 T is the tension of the envelope, and P the internal fluid pressure. This 

 is simply the equation of an elastic surface where T represents the 

 coefficient of elasticity ; in other words, a flexible elastic shell has the 

 same mathematical properties as the fluid membrane-covered egg. 

 The author goes on to discuss particular applications of this equation 

 of equilibrium. 



Development of Polypterus senegalus.* — J. Graham Kerr has 

 worked over the collection of eggs and embryos of Polypterus made 

 by the late J. S. Budgett. The eggs seem to be deposited in shallow 

 lagoons early in the rainy season, and apparently adhere to submerged 

 twigs or water-plants. There is some indication that fertilisation is 

 internal. The young fry apparently accompany a parent (probably the 

 male) in a dense swarm. 



The segmentation is complete, and in its earliest stages nearly 

 equal ; the invagination groove is at first nearly equatorial ; as the 

 curve described by the groove becomes closed, an enormous "yolk- 

 plug " is formed ; rudiments of external gills and cement organs 

 appear at an early stage ; the buccal cavity is for a while a widely-open 

 space bounded by the cement organs, the lower side of the head, and the 

 cardiac region. 



The mesoderm of the trunk region arises as it does in Lepidosiren, 

 Protopterus, and Petromyzon, by "delamination." A well-developed 

 solid post-anal gut is present, which eventually breaks up and disappears. 

 It is interesting to find that the secretory epithelium of the cement 

 organ is endodermic, arising as a pair of hollow enteric diverticula, 

 which become cut off from the rest of the endoderm and establish a 

 connection with the outer surface. 



The lung rudiment is median and ventral, and very soon shows 

 asymmetry. The pancreas develops from three rudiments, and the liver 

 is really a hepatopancreas, having pancreatic tissue spread out over part 

 of its ventral surface. 



* The Work of John Samuel Budgett (Cambridge, 1907) pp. 195-284 (3 pis. and 

 67 figs.). 



