DEVELOPMENT OF SPELERPES BILINEATUS 185 



some of the early cleavage stages, I have accidentally deflected 

 the course of a furrow. If a spot of stain (described below) be 

 placed on an egg in the path of the advancing furrow, the furrow 

 does not always pass through the spot but may curve around it, 

 following its edges closely just as though the stained spot were 

 a real obstacle which it could not overcome. Such cases lend 

 favor to the interpretation suggested above. 



Internal aspect of cleavage 



The differences among individual eggs make it difficult to give 

 a satisfactory account of the process of cleavage within the egg. 

 Variations are so numerous that I shall describe only such as 

 seem necessary to show its usual course. 



The plane of first cleavage as it cuts into the egg may separate 

 the two halves of the egg material by a considerable space. With 

 further progress, the space closes up by the approximation of its 

 edges as already described. The parts of the egg already cut, 

 that is, the deepest parts of the furrow, remain separated for a 

 time, so that when an egg is sectioned perpendicularly to the first 

 furrow in a vertical plane, a sort of cleavage cavity is formed. 

 But this is only temporary. In some cases at any rate, the edges 

 of the cells come together and actually fuse. A careful studj^ of 

 the sections of an egg similar to the one shown in plate I, fig. 2, 

 from which it differed in showing no furrow whatever except in 

 regions corresponding to the spoon-shaped ends of the furrow, 

 failed to show any separation of egg material except at points 

 where a furrow was visible externally. The lower part of the yolk 

 mass may not be cut through by the cleavage plane for a long 

 time. The progress of the second plane is similar to the first. 



In some eggs at the eight-cell stage, those parts of the micro- 

 meres which lie at the upper pole of the egg become very thin 

 (fig. 13), although they are thicker towards the equator. A 

 vertical section of another egg at the eight-cell stage, cut diag- 

 onally to both of the first two planes of cleavage, is shown in fig. 

 14. It represents the other extreme in variation of microniere 

 shape. In later stages, this last is the common type, only occa- 



