EMBRYONIC DEVELOPMENT 217 



tive. Some of these topics will be considered in connection with concrete 

 illustrations. 



Orientation of Embryo. — The higher animals are all bilaterally 

 symmetrical; yet they all develop from an egg which is apparently radial. 

 The animal pole is differentiated from the vegetative, and it is clear why 

 development should commence in the animal portion. But so far as can 

 be seen in an unfertilized egg, the head of the embryo might be turned 

 toward any point in the circumference of the circle of which the animal 

 pole is the center. What decides the position which it 

 actually does take? 



In the frog, the median plane of the future animal 

 is fixed by the point of entrance of the spermatozoon 

 in fertilization (Fig. 190). The first cleavage of the 

 egg passes through that point and also thi'ough the 

 animal pole and vegetative pole. Up to the time of Fig^~790 Sec- 



fertilization, any plane passing through the two poles tion through frog's 

 may become the plane of symmetry. In some of the |^J^ cleavag!J!Thow- 

 salamanders either the first or the second cleavage ing at right the 

 plane may become the median plane, and the entrance T!^ ^ ^ J'o + ^^^-.T!!^ 

 of the spermatozoon has nothing to do with fixing the (Modified from 

 positions of these planes. In fishes, sea urchins, '^ " ^^ 

 and some other animals there is no connection between the early 

 cleavages and later symmetry, and in them it is unknown how the median 

 plane is determined. 



After the position of the embryo is fixed, all later questions of orienta- 

 tion are settled in relation to it. When, by artificial methods, a second 

 embryo is made to develop at the surface of the same egg, it is roughly 

 parallel with the first, with its head pointing in the same direction. A 

 patch of ectoderm in which gills would normally develop at its anterior 

 margin may be cut out, turned halfway around, and made to grow in 

 place. The gills still grow in the anterior portion, but this was originally 

 the posterior part. Also, if the regenerating stumps of cutoff arm and leg 

 rudiments be removed and their positions exchanged in transplantation, 

 the anterior one becomes an arm, the posterior one a leg, which is the 

 reverse of their normal fate. 



Some biologists have suggested that a gradient of some sort is set 

 up at the first orientation of the embryo. Perhaps a chemical substance 

 occurs in gradually less and less concentration from front to back, or a 

 physical phenomenon becomes less and intense in that direction, and 

 the position of structures is governed by this gradient. Little is known, 

 however, that would establish this supposition. 



Principle of Determination. — Another important question is why dif- 

 ferent parts of an embryo produce different structures. In the majority 



