A SUMMAR V OF EMBR YOL O GICAL DE VEL OPMENT. 25 



Unguiculates, we shall have occasion to refer to the Insectivora, of which the mole 

 may serve as a type ; the Cheiroptera, or bats ; the Rodents, including the rats, 

 guinea-pigs, rabbits, etc.; the Carnivora, cats, dogs, and allied animals; and, 

 finally, the Primates, which include the lemurs, monkeys, apes, and man. 



Of the Ungulates, we shall have occasion to refer chiefly to the pig and the 

 sheep. The following table presents these animals which we shall have occasion 

 to consider in their proper order. 



Of the invertebrate animals there will be little to be said. There are two 

 types of invertebrates which show relationship in their structure to true verte- 

 brates. One of these is the class of jointed worms, or Annelids; the other is the 

 class of Atriozoa, which comprises the subdivisions of Tunicata and of the Ceph- 

 alochorda. All of our observations on the development of this last type are based 

 on the one genus, Amphioxus, which will therefore be the name which we shall 

 use whenever we have to refer to these animals. 



Definition of Anlage. 



There will be frequent occasion to use this word in a strictly technical sense- 

 It has been adopted from the German, as there is no satisfactory English equiva- 

 lent for it. The French use the word "ebauche," and the Italians " abozzo." 

 Attempts have been made to introduce Greek derivatives or other terms, but 

 they have not met with success, so that " Anlage " is now used very widely both 

 in America and in England. It may be defined as follows : The first accumula- 

 tion of cells in the developing embryo recognizable as the commencement of a 

 structure, organ, or part. 



A Summary of Embryological Development. 



The following summary applies to what is known of vertebrates only. It 

 would require some modifications to be applicable to the whole animal kingdom. 

 Each individual arises from a single cell which is termed the impregnated or fer- 

 tilized ovum. From this all embryological study starts. The fertilized ovum 

 has its earlier history, since it is the product of the fusion of two sexual elements. 

 It is a living cell, and therefore contains protoplasm and nucleus. It is also 

 furnished with a certain amount of material known as yolk, which exists in 

 the form of separate granules imbedded in the protoplasm. This yolk is the 

 reserve food material, and by the assimilation thereof the pr9toplasm of the 

 ovum can grow. 



The first step in development is the repeated division of the original cell so 

 that there is produced an increasing number of cells. The earlier stages of this 

 cell multiplication are designated as the segmentation of the ovum. This name is 



