THE SNAIL 83 



The ova of those gastropods which are not abundantly 

 furnished with food yolk are divided by two successive con- 

 strictions into four large blastom'eres of nearly equal size, and 

 these by three successive processes of unequal division give 

 rise to twelve smaller blastomeres which lie like a cap on the 

 upper pole of the embryo (fig. 20, A). The smaller cells are 

 called micromeres and the larger macromeres. The micro- 

 meres increase by division, while the macromeres remain in- 

 active, and the result is a hollow sphere or blastula with a toler- 

 ably large segmentation cavity or blastoccele. The roof of the 

 blastula is formed by the micromeres, the floor by the four 

 macromeres. The macromeres now divide and form a plate 

 at the lower pole of the embryo, which is presently folded 

 inwards into the blastoccele so that a two-layered embryo or 

 gastrula is produced. During the process of gastrulation two 

 cells derived from one of the macromeres pass into the 

 blastoccele and lie side by side between the invaginated 

 macromeres and the body-wall. Their position marks the 

 hinder end of the embryo, and a comparison with the develop- 

 mental history of Lumbricus shows that they agree in origin 

 and position with the two large mesomeres which are formed 

 at a corresponding period in that animal. They are^ in fact, 

 the primary mesoblast cells or mesomeres of the gastropod, 

 and they give rise by repeated unequal divisions to two rows 

 of cells, the mesoblast bands, extending forwards between the 

 outer layer or epiblast and the inner layer or hypoblast of the 

 two-layered embryo. During the process of gastrulation (or 

 even during the blastula stage in the limpet), the epiblast cells 

 situated at the equator of the embryo become larger and more 

 columnar than their neighbours and develop tufts of large cilia. 

 There may be one, two, or three rows of such cells forming 

 a complete girdle round the embryo and dividing it into an 

 upper and a lower hemisphere. In many cases ah apical tuft 

 of cilia is developed at the pole of the upper hemisphere, and 

 there may be lateral subsidiary tufts in its neighbourhood. A 

 similar tuft formed in the neighbourhood of the hinder end of 

 the blastopore in the lower hemisphere is known as the anal 

 tuft. In the first stages of gastrulation the gut opens by a 

 wide gastrula mouth or blastopore at the lower pole of the 

 embryo. As development proceeds, the blastopore becomes 

 narrowed and sHt-like, and is carried by the unequal growth of 



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