336 GENERAL PRINCIPLES 



tube ultimately gives rise to the central nervous system of the 

 animal. Before the tube is completely formed the edges of the 

 ectoderm slip over it from each side toward the median line 

 and fuse. This superficial part of the ectoderm gives origin 

 to the epidermis with its modifications, and the sense organs of 

 the skin. 



716. The Notochord. — While the medullary plate is form- 

 ing in the ectoderm the notochord and mesoderm are also 

 taking origin from the entoderm. A longitudinal inverted 

 groove is formed by the bulging upward of the entoderm along 

 the mid-dorsal line. The edges of this groove unite to form a 

 tube, which is then cut off from the entoderm. This tube 

 develops into the notochord. 



717. The Mesoderm. — At the same time two series of pock- 

 ets are formed by the bulging outward of the entoderm in the 

 dorsal lateral quarters, i. e., on either side of the notochord. 

 These pockets also close and are cut off from the entoderm. 

 They are called mesodermic somites, and are the first evidence 

 of the segmentation of the body. They gradually extend down- 

 ward and upward till they finally completely surround the 

 medullary tube, the notochord and the remaining entoderm. 

 The cavities of the mesodermic pockets develop into the body 

 cavity. 



718. Other Types of Cleavage. — So far as the segmentation 

 stages are concerned the chief deviations from the lancelet 

 type may be ascribed to the quantity and disposition of the 

 yolk. In the lancelet egg, cleavage is total and equal. In the 

 frog's egg there is a large amount of yolk accumulated largely 

 at the vegetative pole. In consequence cleavage, though also 

 total, is unequal and at the first horizontal division the cells of 

 the vegetative pole are many times larger than those of the ani- 

 mal pole. In the eggs of Cephalopods, many Fishes, Reptiles 

 and Birds, the quantity of protoplasm is small compared with 

 the yolk and forms a thin layer at the animal pole. When 



