568 



CONTINUITY OF LIFE 



cles are clearly marked off, as is also the 

 optic cup which will become the retina of 

 the eye. The primitive ear is visible as a 

 tiny vesicle (auditory vesicle). The most 

 conspicuous organ is the heart, which is 

 already functioning vigorously. Its early ac- 

 tivity is essential so that the circulating 

 blood can bring food from the yolk mass 

 to nourish the growing embryo. Simple dif- 

 fusion would be hopelessly inadequate to 

 care for the needs of this rapidly developing 



lun^S 



pancreas 



Intestine 



Fig. 23-7. The very young human embryo shows the 

 beginnings of several organ systems. The digestive 

 tract is shown here in black. Note how the lungs and 

 pancreas are forming as outpocketings from the primi- 

 tive gut. The nervous system and the kidney are very 

 primitive at this stage of development (under five 

 weeks). 



embryo. Another important event is tak- 

 ing place at this time, tliat is, the foiTna- 

 tion of the extraembryonic membranes. The 

 embryo is completely enveloped by the am- 

 nion (p. 533). This membrane is formed by 

 a progressive folding over the embryo of 

 the double-layered extraembryonic tissue 

 from either side (Fig. 23-8). The allantois 

 is formed later as an evagination from the 

 hind gut. 



Once the pockets and folds have formed, 

 the growth rate of the various parts is 

 highly variable; some cells grow much 

 faster than others, so that some parts of 

 the organ become greatly emphasized over 

 other parts. For example, in the formation 

 of the liver, the cells which form the bile 

 duct grow relatively little, whereas those 

 that give rise to the secreting portion 

 multiply millions of times, resulting in a 

 massive organ connected to the gut through 

 a tiny tube. Similar unequal growth takes 

 place in the nervous system. It is first a 

 smooth tube, but very soon elaborate folds 

 appear which result from the rapid growth 

 of some cells and the retardation of others. 

 Eventually a highly folded structure is 

 formed that is capable of coordinating the 

 parts of tlie entire body. 



Another method of differentiation during 

 development is cell migration. Cells once 

 formed do not always remain in their origi- 

 nal positions, and during early development 

 individual cells as well as groups of cells 

 migrate a great deal. The mesodermal cells 

 especially, either as single cells or small 

 groups, break loose and wander through 

 the tissues to set up housekeeping in new 

 locations where they differentiate into new 

 structures, such as muscles, blood vessels, 

 and all sorts of supporting tissue. This is 

 not a fortuitous migration, for each goes 

 directly to its future location and settles 

 down immediately to the job of building 

 the structure that is essential in the final 

 organism. This is one of the many riddles 

 in embryology. What forces are operative 

 in directing these cells to the proper loca- 

 tion and in causing them to differentiate 

 there into the right kind of cells to do a 

 specific job? For example, how do the tiny 

 masses of nerve cells that migrate out from 

 the primitive nerve tube find their way 

 through "miles" of other cells to become 

 associated with the muscle cells that have 

 arrived at their location some minutes be- 

 fore? There is a question of time as well 

 as space. If the nerve cells arrived before 



