FORMATION OF THE EMBRYO. 783 



formly from the centre outward. The blastoderm accordingly, when 

 completed, is a smooth, even membrane, having the same texture 

 throughout. 



But when the process of incubation commences, the blastoderm grows 

 more rapidly at particular points, and along certain lines of direction, 

 than elsewhere. What may be the determining cause of such a con- 

 centration of growth in special situations, it is impossible to say ; but 

 its result is that the blastoderm, enlarging more rapidly in one direction 

 than another, is thrown into undulations, which indicate, by their posi- 

 tion and size, the unequal expansion of the blastodermic membrane. 

 Thus, if it grow more rapidly at one particular point than in any of the 

 surrounding parts, it will form at that spot a conical eminence or de- 

 pression, according as it meets with less resistance above or below. If 

 a similar rapidity of increase were to affect a considerable portion of the 

 membrane along a transverse line, the consequence would be a transverse 

 fold ; and if the same thing were to occur in an antero-posterior direc- 

 tion, it would cause a longitudinal fold. The subsequent history of em- 

 bryonic development shows continual repetitions of this process, often 

 on a much larger scale than that exhibited in the blastoderm. The 

 folds of the intestinal canal, the valvulae conniventes of its mucous 

 membrane, the convolutions of the brain, and the tubular windings of 

 the perspiratory glands, with many other analogous forms, are pro- 

 duced in a similar way. All these structures are at first smooth or 

 straight. They become thrown into folds or convolutions at some period 

 during the development of the embryo, whenever they grow more 

 rapidly than the surrounding parts. 



Position of the Embryo in the Egg. Although the blastoderm is at 

 first apparently of uniform structure throughout, yet each particular 

 part has from the beginning a physiological individuality, which leads 

 to its subsequent development into a special organ or part of an organ. 

 This is evident from the manner in which the local activity of nutrition 

 gives rise to the appearance of folds, running in definite directions, and 

 determining in this way the future location of the head, the tail, and 

 the sides of the body. But it is manifested still more remarkably in 

 the position assumed by the entire embtyo. The yolk of the fowl's egg 

 has a nearly regular spherical form ; and the cicatricula, as well as the 

 blastoderm into which it is converted, is a circular spot upon its surface. 

 The ovoid form presented by the whole egg, with one round and one 

 pointed extremity, is given to it by the deposit of albumen round the 

 yolk, in the middle and lower parts of the oviduct, after fecundation 

 has taken place. And yet, when the rudiments of the embryo first 

 become peceptible in the area pellucida, it is so placed as to lie cross- 

 wise to the long axis of the egg, with its right side toward the round 

 end and its left side toward the pointed end. The exceptions to this 

 rule are so few as to show that, even before incubation has commenced, 

 one particular portion of the circular blastoderm is destined to become 



