EMBRYOLOGY 381 



Very frequently certain irregularities in division have appeared by 

 this time, and after it segmentation is not so regular, the smaller 

 cells at the animal end divide more rapidly than the larger yolk- 

 laden ones at the other pole. While these divisions have been in 

 progress a cavity has appeared in the centre of the cells, and so very 

 soon we have produced a characteristic hollow sphere, the blastula, 

 composed of a single layer of cells surrounding a central space, the 

 blastoccel or segmentation cavity. This is a noteworthy stage, and 

 recalls the condition in certain forms of colonial Protozoa, e.g. Volvox. 

 The cells at the upper or animal pole are smaller and contain less 

 yolk than those at the lower or vegetal pole. 



The next step is an important one, since it consists in the trans- 

 formation of this embryo, as a developing egg is termed, with but a 

 single layer of cells into one with two layers. This process we term 

 gastrulation, and it occurs in the following way. The lower pole 

 becomes flattened, and then slowly this flattened area folds in and 

 finally, with the obliteration of the segmentation cavity, lies next 

 to the cells of the other pole. The result is a hollow cup-shaped 

 embryo, the gastrula, composed of two layers of cells, an outer 

 covering of small cells derived from the animal pole, and an inner 

 lining of larger vegetative cells. In this way we have formed an 

 internal cavity, the primitive gut or archenteron, which opens to the 

 exterior by a wide circular aperture, the blastopore. The cells lining 

 this cavity are termed the entoderm (or in older terminology 

 hypoblast), and the external layer of cells the ectoderm (or epiblast). 

 Thus the process of gastrulation, resulting in the laying down of the 

 two primary germ layers, as they are termed, has led to the formation 

 of a diploblastic embryo which, in the possession of these layers, 

 of a single internal cavity and of a single external opening, recalls in 

 its essentials the condition permanent in an adult Ccelenterate like 

 Hydra. This likeness is made more obvious when the embryo 

 lengthens and the blastopore becomes considerably reduced. 



While this lengthening is in progress, a strip of ectodermal 

 cells along the middle of the flatter dorsal surface becomes delimited 

 from the surrounding cells and sinks slightly below the level of the 

 remaining ectoderm. This constitutes the neural or medullary 

 plate, and it extends from the upper lip of the blastopore forward 

 almost to the anterior end. The cells bordering on this strip become 

 slightly raised up to form the neural folds or ridges, and these 

 gradually grow together so as to roof over the neural plate, save for 

 a small circular area, the neural pore, at the front end. The process 

 of enclosing the plate at the hinder end is also aided by the upward 

 growth of the lower lip of the blastopore, so that this aperture 

 becomes closed. While the general ectoderm has in this way been 



