330 GENERAL EMBRYOLOGY 



And further, there is little reason for supposing that here too 

 the essential determinative conditions are other than nuclear, 

 although from the nature of the case, the evidence must be 

 less direct. 



We shall limit ourselves in another direction also. It is 

 obviously impossible to give a brief, and at the same time an 

 adequate, account of the extremely diverse methods of gas- 

 trulation, germ layer formation, etc., in the Metazoa as a whole. 

 We shall, therefore, confine ourselves here largely to the con- 

 sideration of these events among the Chordata. This will 

 enable us to give a more adequate consideration to the topics 

 selected. 



We have seen that while no definite termination can be 

 placed to the period of cleavage, there is rather general, though 

 arbitrary, agreement that cleavage may be said to have termi- 

 nated when the blastomeres become arranged as a more or 

 less definite epithelium or layer, bounding an internal space of 

 some sort; and further that this may, in some cases, also be 

 marked by the attainment of a certain nuclear-cytoplasmic 

 relation. The organism exhibiting these characteristics is 

 known as the blastula. In this stage the organism is essentially 

 a monodermic structure, that is, it is composed of a single tissue 

 or layer of somewhat similar cells. In the simplest form of 

 blastula this layer is but one cell thick (Amphioxus, Fig. 150, A), 

 but in most of the Chordate blastulas the wall is many cells in 

 thickness. 



Like the cleavage pattern, though to a much greater extent, 

 the general form of the blastula is largely determined by the 

 amount of yolk or deutoplasm contained in the ovum; and the 

 form of the blastula, in turn, largely determines the form of the 

 gastrula, and the methods of gastrulation and germ layer 

 formation. We may for convenience, therefore, distinguish 

 three general forms of blastulas, although intergradations are 

 not infrequent. When the ova are nearly homolecithal, and 

 cleavage is total and adequal, as in Amphioxus (Fig. 150, A), 

 the blastula is practically a hollow sphere (coeloblastula) . Its 

 wall is a simple epithelium, one cell in thickness, and its cavity, 



