2 INVERTEBEATA CHAP. 



this history is thoroughly known are very few. Hence a careful study 

 of these few cases must suffice to include the certain and indisput- 

 able results of the science of embryology. Outside this limited field 

 are a number of suggestive facts from which no certain conclusions 

 can be drawn, but which serve as clues to indicate those lines 

 for future research which will probably give the most interesting 

 results ; some of these will be referred to in the following pages. 



The germs from which animals spring are of two kinds (a) those 

 that can develop directly, or asexual germs ; and (Z>) those that under 

 ordinary circumstances are incapable of development until they have 

 united or "conjugated" with another germ. These latter are the 

 sexual germs, and in all the animals which we shall have to consider 

 they are unicellular, and are of two kinds, viz. comparatively large non- 

 motile germs or ova (eggs), and small motile germs or spermatozoa. 

 The organism which produces the ova is termed the female, that 

 producing the spermatozoa, the male, and that which results from the 

 union or conjugation of these two types of germ is called a zygote. 

 If, as often happens, both kinds of germs are produced by the same 

 adult, this is termed a hermaphrodite. In this case, however, the 

 spermatozoa usually ripen before the ova and the animal is said 

 to be protandrous. A case where the eggs ripen before the male 

 germs is very rare amongst animals, but is commoner amongst 

 flowering plants, such an organism being termed protogynous. 



Most asexual germs are multicellular, containing several nuclei 

 and in some cases portions of more than one tissue of the parent. 

 Such germs are called buds, and the laws of bud development are as 

 yet most imperfectly known, but in many cases they seem to differ 

 markedly from those governing the development of the zygote. In 

 this book our attention will be directed mainly to the development 

 of the zygote, but the most important facts about bud development 

 will also be given. Some asexual germs are called parthenogenetic 

 ova, because in development and appearance they resemble true ova, 

 from which they differ only in being able to undergo complete 

 development without conjugating with spermatozoa. The develop- 

 ment in this case is the same as that exhibited by true ova. 



The development of ova and of spermatozoa, from their first 

 appearance until they become capable of union with one another, is 

 known as gametogenesis, since gamete is a convenient word to 

 designate both kinds of sexual germ. It is called oogenesis for the 

 ova and spermatogenesis in the case of spermatozoa. 



Both forms of germ when first distinguishable are small, rapidly 

 dividing cells which are often termed primitive germ cells. In 

 these divisions the nuclear substance, which absorbs stain the 

 chromatin becomes arranged in the form of a definite number 

 of rods or chromosomes denoted by the symbol 2x. The actual 

 number is characteristic of the species in question, and it is often 

 assumed, and is taught in most text-books, that this number is 

 characteristic of the nuclei of all the cells in the body when they 



