COMPARISON OF LARVAL AND EMBRYONIC TYPES. 17 



greater the number of cells of the undifferentiated type that can be produced, the 

 greater will be the number of elements which can be later differentiated. 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 arise from the 

 segmentation of the ovum by the term "embryonic cells. 1 ' 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 or other devices 

 from their enemies. Their only task is to pursue their own development. Under 

 these circumstances it is possible for the embryos to continue for a long time the 

 production of embryonic cells, and we observe that the beginning of the differen- 

 tiation 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 sufficient 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 afiimals and develop most perfectly according to the em- 

 bryonic type. This peculiarity is due to the fact that two special physiological 

 devices have been evolved in the mammals to supply food to the developing em- 

 bryo. First, there is a special relation established between the embryo and the 

 uterus by means of a complicated adjustment of embryonic and uterine tissues, 

 which supplies nutrition to the embryo from the blood of the mother. Secondly, 

 there are the mammary glands, which also serve the same function. By these two 

 devices the embryo is even more completely freed from ' the necessity of seeking its 

 food and protecting itself than is the case with those forms, such as the birds or 

 elasmobranchs, in which the supply of food material is very large. 



