The American Naturalist. 



EMBRYOLOGY. 1 



Experimental Embryology. Recent numbers of Roux's 



Archiv fur Entwickelungsmechanik contain numerous additions to our 

 knowledge of the possibilities resident in the early stages of the devel- 

 opment of animals, possibilities unsuspected till direct experimental 

 interference made them evident. 



T. H. Morgan of Bryn Maur presents evidence 8 to show that two 

 blastulae of the sea urchin, Sphsereehinus, may fuse together and form 

 one embryo. When eggs are shaken just after fertilization they may 

 loose their membranes and afterwards some of the resulting blastuhe 

 are found to have twice the normal size though otherwise like the usual 

 blastulae in appearance. Such large blastulaj are stated to arise from 

 the fusion of two common blastulse. 



Notwithstanding this complete fusion the future development of such 

 enlarged blastulae gives evidence of their dual origin. At the gastrula 

 stage two invaginations are formed. 



One may be much the greater and the two may not appear at the 

 same time. The two invaginations stand in no fixed relation to one 

 another and may appear in all parts of the compounded blastula. 



Later the larva that develops from two fused blastula? tends to 

 develop two sets of arms and two systems of skeletal rods, but those 

 accompanying the lesser invagination are much reduced in size and 

 less perfect than the rods associated with the main invagination. 



A second paper 3 by the same worker records a variation in the 

 cleavage of the above sea urchin when some of the eggs were shaken. 



While most of the eggs divide into 2, 4, 8 and 16 cells some were 

 found to divide at once into three. These 3 cells are elongated parallel 

 to the planes that produced them. When they next divide they all do 

 so lengthwise, in flat contradiction to " Hertwig's law." These six 

 equal cells lie in a plane at right angles to the two cleavage planes 

 that have produced them. 



Such eggs may develop into gastrula?. They form six small cells or 

 ie pole of the mass in place of the normal four. The 

 • thinks " a micromere field must have been present in the egg 



