30 EMBRYOLOGY 



half of the original egg. As Figure 11 shows, each of these halves may be 

 fertilized. The animal half when fertilized undergoes cleavage and develops 

 into a permanent blastula. This half reaches the blastula stage and goes no 

 farther, and thus no endoderm forms. The blastula develops long cilia, swims 

 about for many days, and then dies. The vegetal half, on the other hand, 

 undergoes cleavage and forms a partial larva which contains a digestive 

 tract composed of endoderm, some ectoderm, and a skeleton formed by 

 mesoderm cells. A loose aggregation of mesoderm cells is called mesenchyme. 

 The ectoderm becomes ciliated so that the larvae swim, but further develop- 

 ment is blocked. 



Neither of these two types of larvae can survive. Each lacks something. 

 The greater lack is in the animal half, which develops no further than the 

 blastula stage. The vegetal half will form a gastrula and a partial larva. The 

 location of the egg nucleus makes no difference in this type of operation. It 

 may be in either the animal half or the vegetal half without modifying the 

 results in any way. 



The above experiment bears out the observations on the behavior of the 

 pigment band during development of the whole egg and again shows that 

 the animal half forms ectoderm but no mesoderm or endoderm. The vegetal 

 half forms chiefly mesoderm and endoderm, but it may also form some 

 ectoderm. 



Now we would like to know how this organization with respect to the 

 three germ layers can be established in the egg itself. What parts of the egg 

 are responsible for organization ? Some information pertaining to this ques- 

 tion may be obtained by centrifuging eggs at high speeds. If we take the 

 unfertilized egg of a certain sea urchin and expose it to a high centrifugal 

 force we can shift the pigment granules into any region of the cell we wish. 

 In Figure 12 the pigment is thrown into one side of the egg by the centrifugal 

 force C,C X . The rest of the egg becomes stratified into the various layers of 

 protoplasm. The granules, being heavier than the rest of the protoplasm, 

 move toward the outer end of the centrifuge C v Toward C, the opposite 

 end of the centrifuge, the lightest particles in the egg will accumulate; these 

 are the fat droplets containing lipids. They thus go to the light pole of the 

 egg whereas the pigment granules go to the heavier pole of the egg. And 

 in between the lipids and the pigment granules are granules of various sorts 

 and sizes, taking up about half of the egg. The nucleus comes to lie in the 

 light half of the egg, along with the lipids, and this leaves a region of very 



