INTRODUCTION. 



IX 



and an intermediate substance not affected by staining fluids. The nucleus network 

 goes through a definite series of changes, and finally divides into two equal or sub- 

 equal masses, which retreat from one another, and go through, in inverse order, the 

 changes undergone by the mother nucleus, finally forming the nuclei of the two 

 daughter cells. The cell-body divides after the young nuclei have separated from one 

 another, but before they have assumed the characteristics of quiescent nuclei." Cell 

 division, he adds, probably exhibits periodicity, going on vigorously at certain times, 

 and but little, or not at all, in the intervals. 



Tissues. 



Although we shall treat of the development of animals on a subsequent page of 

 this introduction, at the risk of repetition, we will give here an epitome of the earlier 

 features common to the development of all animals above the Protozoa as an introduc- 

 tion to the subject of tissues. 



In all the animals above the Protozoa, besides reproduction by fission and bud- 

 ding occurring in the lower groups, there is a sexual reproduction by which one animal 

 or one part of an animal produces an egg, while another produces the male element or 

 spermatozoan. These two unite and produce the fertile egg, which in its turn is con- 

 verted into a perfect animal like the form from which the genital products arose. In 

 only a few forms will the egg develop without an intervention of the male sexual ele- 

 ment, and these cases of par- 

 thenogenesis, so far as at pres- 

 ent known, are confined to the 

 rotifers and arthropods. 



The egg is to all intents and 

 purposes a simple cell, and the 

 processes by which it forms the 

 complex adult are but those of 

 cell-division, the resulting cells 

 becoming specialized, some for 

 the performance of one function, 

 some for another. In the typi- 

 cal form of this cell-division or 

 segmentation, the egg divides 

 first into two equal parts or cells, 

 each of these again into two, 

 producing four in all, and so on, 

 the regular numbers being, 2, 4, 

 8, 16, 32, 64, etc. This regu- 

 larity is but rarely perfectly car- 

 ried out ; the exceptional forms 

 will be mentioned later. The 

 result of this segmentation is 

 the production of a more or less 

 spherical body, which has received the name morula, from its resemblance to a mul- 

 berry (Fig. II. E.) Usually this morula is at first solid, but it increases in size, the 

 result being a hollow globe (Fig. II., F, surface view ; G, section), the blastula, the 

 vacant centre being known as the segmentation cavity. The next step is the con- 



FiG. II. — Early stages of development of Monoxenia; A, egg after 

 disappearance of the nucleus; B^ egg with nucleus; G, lirst seg' 

 mentation; D, second segmentation; E, morula; F, surface view, 

 and G, section of blastula; H, section of early gastrula. 



