DEVELOPMENT PREVIOUS TO THE SWIMMING STAGE 19 



The third cleavage is restricted to a smaller mass than the first and 

 second, and also appears to be subject to a greater variation. It may 

 occur that the two descendants of the smaller of the first two blastomeres 

 divide, or it may be that division is at first confined to only one of these, 

 viz., the middle one of the three most completely detached from the deuto- 

 mere (Plate V, fig. 14) , Irregularity may even begin in the second cleav- 

 age, in that the larger of the two original blastomeres does not keep pace 

 with the smaller one. In any case segmentation advances most rapidly 

 among the descendants of the first micromere, followed closely by those 

 detached from the first macromere. The blastomeres can not preserve a 

 horizontal plane, but curve over the deutomere (Plate I, figs. 5, 6), so 

 that it soon becomes impossible to keep track of the planes of cleavage. 

 The polar bodies and the deutomere serve as landmarks for orientation, 

 and it is possible, in a general way, to observe the direction and manner 

 of progress. Although the first cleavage began with an equal division, 

 the result at the end of the succeeding rest was a large macromere and a 

 small micromere resembling a bud. Similarly the second cleavage ef- 

 fected the division of the micromere and the formation of another bud. 

 In this way the deutomere becomes reduced, and the number and extent 

 of the small blastomeres increased until the latter form a cap over and 

 partly surrounding the ever diminishing deutomere, which, after a time, 

 is withdrawn into and almost enclosed by the cap, where it finally divides 

 into two equal cells (Plate V, fig. 19). 



Embryonic Stages. — With the series of cell-divisions and the 

 simple arrangement of the blastomeres the resulting structure has come 

 to be a very different object from the solid sphere vv^ith which it started. 

 From the time of the first cleavage it is no longer an egg, nor an oosperm; 

 it is in the strict sense an embryo. At first the cells are everywhere close 

 to one another, soon forming the stage corresponding most nearly with 

 what has been called the morula or mulberry mass in many other animals. 

 By mutual pressure each cell tends to keep to the surface, occasioning 

 more or less of a space in the centre of the mass, but never forming a 

 typical blastula (blastosphere) . The remnant of the deutoplasm presses 

 into and occupies most of this space, where it divides first into two, but 

 soon into a few cells, that are larger than those on the surface. They 

 arrange themselves into a layer arching inwards with a depression open 

 to the outside on the lower, somewhat flattened surface of the whole 

 structure, which is now a gastrula (Plate I, fig. 7; Plate V, fig. 19, 20). 

 The cap of small cells forming a single layer on the outside constitutes the 

 ectoderm (epiblast); the inwardly arching layer of larger cells is the 

 endoderm (hypoblast); the space between the two layers is the segmen- 

 tation cavity (cleavage cavity, primary body cavity, von Baer's cavity, 

 blastocoele) ; while the depression on the under surface is the widely open 

 gastrula mouth (blastopore). With the formation of this double layer of 



