528 



THE DEVELOPMENT OF THE INDIVIDUAL 



The "white of the egg" and the shell are merely envelopes that surround 

 this zygote. The nucleus containing the chromosomes is found in a 

 small germinal disc which lies on the surface of the yolk. After the 

 first mitosis, the cleavage furrow forms between the two nuclei and then 

 stops. The great mass of yolk beneath seems to be too great a barrier 

 for the furrow to penetrate. As divisions of the nuclei continue, how- 

 ever, cell membranes are formed beneath and, as a result, we have a 

 group of small cells on top of the yolk. This group of cells is called the 

 blastodisc. Soon the blastodisc pulls away from the yolk in the cen- 

 ter, but remains attached around its margin and forms something like 

 a blister, the blastoderm. This produces a cavity between the blasto- 







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Fig. 33.4. Partial cleavage of the chick zygote. These diagrams show only a small 

 portion of the large yolk of a fertilized egg. The first cleavage furrow does not extend 

 through this great mass of yolk, but remains only on the surface separating the two 

 nuclei. As the nuclei continue divisions, cell membranes are formed beneath as well 

 as between the nuclei and a group of cells (the germinal disc) on top of the yolk results. 



derm and the yolk which is the blastocoel. We now have the equivalent 

 of the blastula. Next, the blastoderm separates from the yolk at one 

 point and this forms the blastopore. The endoderm now appears as 

 cells invaginate at this region, or they may proliferate directly from the 

 ectoderm. 



In the posterior part of the blastoderm, the ectoderm thickens and 

 forms an area known as the primitive streak. There is a primitive node 

 at the anterior end of this streak which lays clown the cells which form 

 the notochord. This node regresses posteriorly as the cells of the noto- 

 chord are laid down so that when the notochord is formed it lies anterior 

 to the node. Then the ectoderm thickens above the notochord and forms 

 the neural plate which is to produce the brain and spinal cord. Only 

 the central portion of the blastoderm develops into the embryo — all the 

 rest developing into the four extraembryonic membranes which will be 

 described later. 



