LARVAL AND EMBRYONIC TYPES OE DEVELOPMENT. 33 



many and varied directions. The more they become speciaHzed, the more their 

 possibilities of further varied development are decreased. It is thus obvious 

 that the greater the number of cells of the undifferentiated type that can be pro- 

 duced, the greater will be the number of elements which can be later differen- 

 tiated. Hence, the more the period for the production of undifferentiated cells 

 is prolonged and the commencement of differentiation postponed, the more 

 complex may be the degree of organization ultimately attainable. 



It is convenient to designate the undifferentiated cells as they asire from 

 the segmentation of the ovum by the term " embryonic cells.'' The object of this 

 section is to point out that the larval type of development is less favorable for 

 the multiplication of embryonic cells than is the embryonic type; and, further, 

 that the embryonic type becomes more and more marked as we ascend in the 

 animal kingdom. 



The Larval Type. — In the lower multicellular animals we encounter only 

 larvae; sponges, jellyfish, starfish, and worms all pass through their early stages 

 as larvae. Now, larvae are animal forms which have to obtain their own food 

 and to protect themselves against enemies. They are, therefore, provided with 

 a variety of organs, or, as we may say, with differentiated tissues which enable 

 them to perform the various physiological functions which are necessary for the 

 maintenance of their existence. The differentiation of tissues comes in very 

 early. 



The Embryonic Type. — True embryos arise from eggs which contain a more 

 or less considerable amount of yolk or nutritive material, the presence of which 

 renders unnecessary any activity on the part of the embryo to obtain its food- 

 supply; and we find, moreover, that these embryos are protected by hard shells 

 and other devices from their enemies. Their only task is to pursue their own 

 development. Under these circumstances it is possible for the embryos to con- 

 tinue for a long time the production of embryonic cells, and we observe that the 

 beginning of the differentiation proper is correspondingly postponed. The 

 transition from the larval to the embryonic type is very gradual. The yolk 

 appears in the lower animals in small quantities, increasing in some of the higher 

 types and attaining its maximum in some of the highest. Since the embryo is 

 dependent on the yolk, and since the yolk exists only in the higher forms in suffi- 

 cient quantities, it follows that fully typical embryos can occur exclusively in the 

 higher animal types. 



In the mammalia the ovum contains a rather small quantity of yolk, yet 

 the mammals are the highest animals and develop most perfectly according to 

 the embryonic type. This peculiarity is due to the fact that two special physi- 

 ological devices have been evolved in the mammals to supply food to the devel- 

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